ReFX TBL is also wicked for acid, as it has built distortion FX, but i can never get it in tune, and the fine tune control is not accurate enough to get it right.
Another good one is the Muon Tau, if you use it with a distortion plugin.
The best 303 emulator iv'e used though is Rebirth (as mentioned above), iv'e heard that the Bass Station (VSTi version) by Novation is good too :)