Could you kindly suggest an external flash for 1000d within 100$ to 150 $. My main interests are portraits and wedding.
I've got a Lumopro LP160. This is a nice flash for manual-mode use - it's got power comparable to the 580EX, and a rotating, tilting zoom head. It works as an optical slave, with a mode that ignores TTL pre-flashes. I believe this flash unit is a favorite of the strobist crowd, as they tend to favor manual flash settings, anyway. I paid $160 for mine -- $10 over your budget.
If you prefer manual flash, you can try Yongnuo YN-560, or the newly released YN-565EX speedlite. The YN-565EX is said to be a duplicate of the famous Canon 580EX II and is pretty much a 580EX II without HSS and master capabilities but it can be a ETTL or optical slave. A lot of photographers here use this flash and due to its low price, they're very popular among starters.
These flashes might require a little more getting-used-to comparing to Canon branded units, but they are cheap and let you learn the hard way. Also if you ever decide to go pro and buy the 580EX II, you can always use the YN-565 as a slave.
Try the Metz MZ36, a well respected budget brand. It has E-TTL, which means it can use exposure information taken Through The Lens to work out how much flash is needed. You can also tilt the flash head up to bounce it off the ceiling, essential for avoiding blown out faces.
I am a big fan of the Vivitar 285.
You might be able to get them for under USD 100 (amazon price today is about 90 bucks).
I have 2, which I have been using for a couple of years now. I use them in conjunction with Catcus 5 radio triggers.
The only annoying thing about them is that they have full power, 1/2, 1/4 and 1/16. 1/8 seems to have mysteriously disappeared!
Still, they're wonderfully robust, reliable, cheap and reasonably powerful.
I also like the twist-to-set power knob, which (to my mind) is much easier than pushing an up or down button several times to get the desired power setting.
It would be at the high end of your budget, but I'd recommend getting a used 430EX.
Yes, you can get more features with a Yongnuo, and you can get cheaper with an all-manual flash. But if you plan to shoot events, you want TTL capability, so flashes like the Vivitar 285HV or LP160/LP180 or Yongnuo YN-560 are not your best choice. Those flashes are much better choices as 2nd, 3rd, or 4th flashes in an off-camera set-up. Event shooting typically requires a TTL-capable flash on-camera. TTL is like having Av/Tv modes on the camera. When you've only got one chance to grab the shot, the speed of automatic adjustments makes a difference.
A 430EX may not be the most powerful or versatile flash, it may lack some features from the 3rd-party cheapies, but the build quality, component quality, copy consistency, and compatibility are pretty much guaranteed. It will also have some resale value, has good availability on the used market, and you know where to get it serviced if there's a problem with it.