Canon Speedlite 580EX TTL Flash

I'm very happy with the 580EX (original version). It performs well with my Canon 40D, providing enough power in almost all indoor situations and strong enough to be useful for outdoor fill as well.

The standout features of this flash:

1. Acts as a master controller for other Canon flashes.
2. Bounce and tilt. Don't get a flash that doesn't do both or you can't take a vertical orientation shot and still bounce off a ceiling, or horizontal shot off the wall. This is the single biggest reason to get a separate flash - the quality of light is greatly improved by bouncing.
3. Wheel on back for changing manual level, or +/- EV in auto modes.
4. Recharges quickly.
5. Manual focus aid works wonders in extremely low light, even without flash.
6. Lots of customizable options, rear curtain sync, etc.
7. Auto zoom handles crop sensor camera (i.e. when set to 50mm on the lens it zoom to what would really be 80mm for more distance).

Negatives:

1. No sync socket. Requires a Canon TTL cord or other more expensive option to get right off the camera. That said the TTL cord can be had for under $30 on eBay.
2. Relatively large - bigger than a compact camera.
3. Not the cheapest flash around...

One thing people will say is that you should get a 580EX as your first flash so you have a master. I did it the other way around and got the 430 first. When you only have one it doesn't really matter, and the 430EX is cheaper. I really liked the controls on the 580 better, though, and to me it was worthwhile. I also find the rotation on the head better with the 580EX, the 430 only goes 90 degrees in one direction so you have to go the 'right' way around to shoot stright back, something you don't have to worry about with the 580EX. Minor to be sure, but just makes it a little bit less fussy when you are taking shots. Otherwise the 430EX and 580EX are very similar - the 580 being a little more powerful and quicker to use.