There are several very nice aftermarket lenese out there you might consider as well.
The Tamron 28-75 f2.8 runs about $350 or so and is very sharp and has a great reputation. The Sigma 24-70 f2.8 is also a very sharp lens, though a bit heavier. I have one and it performs very nicely. Costs about $400.
For wider angle shots, you might consider the Tokina 12-24, which costs about $500-550. Has garnered a good reputation in the few months it's been out there.
Where things get expensive is the telephoto zooms. If you want a GOOD one, you're going to have to spend at least $500---you can get a Canon 70-200 f4L used on places like Fred Miranda forums for around $500. You can also find deals on used Sigma 70-200 f2.8 lenses for about $650 usually, and those are also very good.
Anything longer than 200, to get a good lens you're going to have to spend serious bucks. The best buy in a long telephoto zoom is probably the Sigma 50-500, which can be found used for under $800. The best buy in a telephoto prime is without a doubt the Canon 400mm f5.6L, which can be found used for under $900. I own one and it's an incredible bargain---sharpest, clearest, most useful lens I own.
Hope this helps.