When trying to get started with flash photography, there are a lot of new options, features and techniques that a photographer gets thrown in to. When starting on a budget, what features will give a flash the most bang for the buck for people just starting out in flash photography that don't have a lot of money to spend?
First and foremost, sufficient flash power is necessary. Flash power is measured in guide numbers. With guide numbers higher is more powerful. At a given ISO and focal length, the guide number is the aperture times the distance to the subject. For example, a guide number of 100 should be able to expose a subject at f/4 that is 25 feet away. It's also important to make sure that you compare apples to oranges with guide numbers that are using similar ISO and focal length numbers.
You also want to look for a flash that has a tilt (and preferably swivel) head. It's very important to be able to send the flash where you need it. Many techniques like bumping flashes off ceilings and walls will give a much more professional look, but also require the ability to send the flash off angle. This is a very valuable feature for a flash to have.
Another consideration that is probably a little less critical (and can be adjusted for) is flash dispersion. How well does the flash spread out the light? If the light is too concentrated, then photos up close will have portions of the image underexposed and other parts overexposed. You can fix this with flash diffusers, but they will also cost you some flash power (typically anywhere from 1/3 stop to 2 stops of power.
You will also want to consider what kind of metering to use if any. If you photograph mostly scenes where you can redo shots and carefully setup your lighting, then metering may be something you can skimp on in favor of other features, but you'll have to either meter your flash with an external meter or work it by hand and some trial and error.
On the other hand, if you are going to be shooting things that you only get one chance on or can't be taking the time to experiment with manual adjustment, then at-least some form of metering is a must. You can get a flash with an integrated meter that you can manually tell it what settings you are shooting on with the camera and do a test flash and it will adjust the flash appropriately for you. The main draw back is it requires a flash exposure prior to taking the shot since the camera and the flash still aren't talking to each other (other than to say the shutter was pressed).
In most cases, just about any hotshoe flash should recognize the shutter release signal and if external metering or manual settings are used, just about any hotshoe flash should work on just about any camera since the shutter release signal is pretty universal. The main side effect of using off-brand flashes though is that TTL typically won't work.
TTL stands for Through The Lens metering. In a TTL setup, the process is actually pretty similar to an externally metering flash, but it happens MUCH faster and uses the actual sensor within the camera for metering (so that filters, optics, etc are taken into account.) When you take a shot in TTL mode, the camera tells the flash all the settings being used for you, it then fires off a pre-flash, checks the exposure and adjusts accordingly for the actual exposure, the actual exposure then automatically occurs with all the factors taken into consideration. The entire process happens so fast it normally appears to be a single flash. It is a very nice feature to have, but also one that can be given up in a pinch in favor of external metering, particularly if you don't make much use of filters that reduce light captured by the camera, but if you can afford it, it is very nice to have. Also, as mentioned before, in most cases, TTL rules out other brands of flash (some third parties have reverse engineered it) since each camera manufacturer uses their own proprietary signals for TTL.
Moving more into the realm of nice to haves, having a sync cord hookup is useful for being able to reuse a cheap flash down the road once you start getting more into flash photography and want to upgrade. One of the great things about flash photography is that it is very common to use off camera flashes. If your current flash today can be your off camera secondary flash tomorrow, it can save your investment from finding it's way on the the trash heap quite so soon. If you are on a really strict budget though and expect to have more funds available in the future (say a student) then it may not be worth investing in this feature over having a higher guide number or a better level of metering.
One final thought is that you also want to consider the cycle time for the flash. The electronics in all flashes are not created equal. If you plan on doing mostly studio work, then a cycle time may be less of an issue, but for events or any kind of higher speed shooting, having to wait 5 or 6 seconds for a full power flash can be a lifetime. This is another one that is really dependant on how you expect to be using the flash.
Similarly, the life expectancy of a flash is important to consider if you are going to use it alot. How many flashes can it take before needing new batteries and how many exposures is it rated for before the flash starts losing power, how about before it is dead? Flashes start to lose power relatively quickly. Even a high end flash like the Canon Speedlite 600EX-RT will start becoming less powerful after just a few thousand flashes. Now, that doesn't mean it turns into a pumpkin, but it does mean that the guide number slowly starts slipping down. Cheaper flashes often equals cheaper flash bulbs and the fade off can start after 1000 flashes or fewer. For the occasional flash photographer, this isn't really an issue, but if you plan to work with it a lot, keep in mind that rapid thermal shock cycles are the hardest on a flash (so if you can let the flash cool down between shots, it will thank you in the long run.)
With a normal budget, there are all the things you regularly look for in a flash, but with a low-budget (say sub-$100) flash, you're not going to be finding any TTL units, HSS capability, etc. etc. All the units are likely to be cheap Chinese-manufactured manual-only flashes (say, the YN-460 or YN-560) and the like.
So, these are likely to be best used for off-camera flash in a Strobist set-up, and I'd highly recommend that you don't go down this path for a first or only flash unit. Think of a flash the way you'd think of a lens. It's as basic and transforming a piece of gear as a new lens, and you should cost-budget in the same neighborhood if you want a good one.
What you give up going cheap
Generally, reliability under hard usage is going to be the key deciding factor on going with a cheapie or, say, something a bit higher-priced. If you're just a hobbyist who's only going to do occasional or light usage of off-camera flash, these can be just the ticket. If you're a pro with clients breathing down your neck at two or three jobs a day, then maybe you want to pay for added reliability.
What you give up going manual-only
It's not just TTL you give up when you dump the non-sync pins on the shoe/foot. All the function where the camera has to talk to the flash is gonna go, too.
The marks of a worthwhile cheapie:
Features to study extra hard
A lot of how these cheapies will disappoint or delight depends on how you interpret the specs they give out. They don't lie, but there are a few fudge factors at work, so you may really need to pay attention.
Guide numbers are given for power, but they're often "cheated" by putting the flash on its tightest zoom setting to narrow and focus the beam from the flash to its tightest so that the flash looks more powerful. Nikon measures all their guide numbers with the flash zoomed to 35mm. Find a review site that measures the actual light output, rather than relying on the manufacturer's stated spec. For example, the YN-560 has a guide number that makes it look like it's as powerful as a 580EX. It's not. It's about the same as a 430EX.
Check if you have zoom. And then check if you have manual zoom control as well as Auto. This is a nice-to-have feature, so sometimes it's dropped from the budget models.
Check and see how many manual power settings you get. And if the flash doesn't do Manual power control don't even go there because that's the only way you can control the flash on a cheap manual-only trigger. The more settings you get, the more flexibility you'll have. 1/128 power is probably the best you're going to find. Probably one of the worser ones is the Vivitar 285HV, which only does 1, 1/2, 1/4, and 1/16 (yup. 1/8 is missing. Think of it like having a lens you can't set to f/4, f/8, f/16, or f/22).
While a flash head can swivel, most low-budget flashes will only swivel 270°, not the full 360° you really want for bounce flash, or for aligning a sensor with line-of-sight. Some don't swivel at all (e.g., Vivitar 285HV).
I've seen some really wackadoodle listings here, but you're basically looking to see if a flash has a PC sync connector. The Vivitar 285HV has a proprietary Vivitar connector. Some flashes don't offer a sync connector at all. Consider what triggers you might be hooking up to the flash. Some eBay listings will actually call a PC sync connector a 3.5mm sync port which it's not. And many won't be screw-type connectors (i.e., the connector's held in by simple friction, not a screw thread. Rely on pictures. This is what a PC connector looks like:
Here's where it gets sticky. You may have to google about and do some additional research, aside from reading specs, because you'll see the phrase "optical slave" and a lot of these 3rd party makers don't specify if this means "dumb" optical slave modes (i.e., one that can be set off by any flash burst--even from a P&S camera) or the TTL proprietary wireless communication protocols (e.g., CLS, wireless-eTTL, Olympus RC [which is NOT radio-controlled]). Particularly since they'll call the mode that ignores a TTL preflash "TTL optical slave" and the like. Again, with a bargain flash, it's unlikely you'll be getting wireless TTL communication.
Newer manual flashes are starting to come with radio receivers built-in. Understand that this receiver is likely to only work with a specific transmitter model--probably from the same manufacturer. Do not assume that any radio transmitter will work with the built-in slave on the flash--even if it's the same brand (e.g., the YN-560III only works with the RF-602, 603, and 603ii, not the YN-622).
A manual-only flash will typically have a foot with a single pin on it. That's the sync signal. It won't have the other TTL pins, unless it's doing some form of TTL communication. If a flash is advertised as doing TTL, but only has a single pin on the foot then the TTL is not being done through the hotshoe/foot, but some other way (optical wireless probably: e.g., Yongnuo 560EX).
The most important item is whether or not it has TTL (iTTL, CLS, etc) and if you require it.
Manual flashes are much cheaper than TTL capable flashes. They are recommended by David Hobby, the Strobist. See http://strobist.blogspot.com/2006/03/lighting-101-start-here.html He states that manual control is mandatory for proper flash work, and some branded flashes only work TTL
There are TTL third party flashes. The Strobist discussion group on Flickr has many threads weighing the pros and cons of specific models and brands. http://www.flickr.com/groups/strobist/discuss/
Here are my must have features:
I am not going to get too extensive here and write a novel on this particular subject. What kind of flash will give you the most bang for the buck kind of depends what kind of photography you will be doing. For event photography I would say you would need a flash with TTL. A reasonably priced flash for that would be the Yongnuo 568 EX which handles TTL and high speed sync. This flash will run you about $180. A Canon 580 EX II would run you $500 or more depending on if it's new or used.
If you are doing outdoor or even indoor portraits you can save a lot of money using manual flashes such as the Yongnuo YN-560 or 560 II. You can get the YN-560 for about $30 used and the YN-560 II for around $70 brand new. Both flashes I use regularly and I have found them to be super reliable. Also I can't for the life of me ever see needing more flash output than they provide.
I hope you found this helpful and here is a link with me using them both in real world conditions. Using YN-560 and YN-560 II