Guys, two primes EVERYONE must try atleast once. (Drumbeat roll here....)
1. 21mm Zeiss Distagon
2. 50mm f1.8 Canon EF
(Everyone= anyone with a full frame camera really)
I'll stick a random one on to that list, the Zeiss Sonnar 180 f/2.8, for only $200 (or as low as $100 if you're a good ebayer) including adapter to EF-mount, it really gets you into the idea of über-shallow-DOF for wildlife (best used in combination with a cable-release, tripod, and bird-feeder a few meters away in your backyard). Or a canon 200 f/2.8 for a bit more, but both still a lot lot lot less than you pay for the 70-200 f/2.8L or 300 f/2.8.