Yongnuo do some great Flashguns at budget prices that are compatible with all Canons at budget prices. Over at the Canon Forums at Photography On The net, it's the budget flashgun of choice with all the advanced users recommending it. It has some great reviews on Amazon.
I second ebiggs' advice. I think the Speedlite 430EX II is a fantastic workhorse flash. I've had one for years... those things are consistent and never fail.
