I like the EF 20mm f/2.8 USM and the Sigma 20mm f/1.8 EX. Both can be found used for about $300-$350. I can't think of any better options in that price range.
I can second the recommendation the EF 20/2.8. It's a very nice lens, if you prefer primes. Well corrected and sharp.
I've been tempted to get Samyang 14mm, mentioned above... (I woudn't want to spend what a 14/2.8L II costs for a lens that I'd likely only occasionally use). The Samyang sells under a bunch of different brand names: Bower, Rokinon, ProOptic (Adorama's house brand). The Vivitar 13/2.8 is the same lens, too... even though they call it a 13mm.
When I win the lottery, I'm going to get the TS-E 17/4L