Specifically, I might be taking pictures inside of a house that's for sale. I want to make them look as best as possible.
While I think it'd be nice to have general answers, let me just give you some of my gear, that I think would be the best I have towards photography.
I'm interested in both gear and in general advice.
Architecture photography often involves shooting large buildings and interiors without the ability to move back so wider lenses increase your chance of framing enough to show your subject.
What also happens is that you often end-up tilting your camera upwards which causes converging verticals. The typical and expensive solution to this is to use a tilt-shift lens. There are 3 current Canon tilt-shift lenses. The shift part is is the one that lets you keep your camera level while shifting the optics upwards.
Less expensive options require some creativity. One easy way is to use a stand-alone ladder. You just have to climb up to get the framing you want while keeping your camera level. For exteriors, you can instead climb (or use a conventional way of going up) neighboring buildings (with permission).
If your camera or tripod do not have a built-in level, you buy a 3-axis level for your hot-shoe. These are great because they let you measure tilt independently for each axis.
An ultra-wide angle is very handy as it'll make spaces look bigger. Something like the Canon 10-22 would be ideal, or the Tamron/Sigma/Tokina version would also do.
28mm is very long on a crop body for indoor architecture, and is going to make rooms look small. If you can't get hold of a wider lens then you might be able to get away with using the 28-135 and shoot panoramas to extend the images widthways, but other than that 28mm wont cut it unless the house is huge.
A lot of the time you'll want to blend natural and artificial light in a high-key scheme to give a bright spacious feel. Well done HDR shots can work very well in this regard.
In any case you'll want to stop down for sharpness and depth of field so a tripod is usually required, though you'll sometimes have to get creative with how you use the tripod as backing into corners is frequently called for to get everything in shot.
Stitched panorama photos can add dramatic impact to your indoor photography. They are not difficult to do and have the following advantages:
a) they can give a field of view that no lens can achieve.
b) panorama software can compensate well for convergence problems, making tilt-shift lenses unnecessary.
c) you can produce large prints which are pin sharp.
You will need a panorama head for your tripod and good panorama software. I have used Hugin which has worked very well for me despite the initially steep learning curve. Hugin even does a good job of stitching together hand held shots so on occasion you can make do without a tripod, though there is no doubt that a tripod gives the best results.
As mentioned before, you better do with a wider lens. The 10-22mm is perfect, while a much cheaper alternative, which is still better than the 28-135mm is the 18-55mm kit lens, used at the 18mm side. Normally, indoor pictures for selling properties are not required to be hi-res, so the (not-so-bad) quality of the 18-55mm should be just adequate.
A tripod is a good idea for taking long exposures in small apertures. Then, you may want to use the Av or Manual mode, even when using flash, in order to blend the naturally looking outdoor view which is seen from the windows, especially during the golden hour, with the flash or lamp internally lit space. With a tripod, I think using the apartments light fixtures and not the flash may be preferable.
To shoot interiors a wide lens is key...as said above the wider the better. However...this does not make for good "architectural photography". One very important, key and integral element to shooting architecture is the ability to stay true to reality. Using an ultra wide lens as Matt Grum suggests, "is very handy as it'll make spaces look bigger," but takes you out of the realm of Architectural Photography. Just because it's a picture of a building...
To begin, you have to keep your vertical lines straight. Any wide-angle distortion and you're out. Use a shift/tilt lens to correct. If you're shooting for a magazine, you need BIG PIXELS. Somewhere in the 20's is a good place to start.
Back to your question: thanks to digital you can shoot multiple frames to adjust exposure for high-key elements like windows and lights and shadows. Good news there. Makes your lighting package a lot smaller. Can combine images through clever layer masking or HDR.
Bottom line, if you're using your 28mm, pano's are probably absolutely necessary to get the job done. That or rent an ultra-wide and be okay with heavily distorted images. Good luck.
Your best friend, after the rectilinear ultrawide lens and tripod, for shooting interiors, is going to be off-camera lighting. HDR and a tripod can only get you so far in terms of control of the light. If you really want shots that look like house beautiful interiors in magazine spreads, you're going to have to light like those guys do.
