For primes:
50mm f/1.4 and 85mm f/1.8 are good choices for your budget.
For zooms: as mentioned above. Do note that Sigma may suffer from build issues (peeling finish after a certain period of usage) and optics issues (yellow cast, left and right of lens uneven sharpness).
Also, what focal length are you looking at?
EDIT: Zaren mentioned the Tamron 17-50 f/2.8. That's a great walkabout for 1.6x crop DSLRs, and held in high regard by many photographers. There are even cases of photographers selling off their 17-40Ls for this lens.