give reFX Claw a go (formally known as tbl bassline, I think). It's free, not a demo, and so I dont think there's any crippling like periodic audio drop out.

The actual synthisis can sound a bit weak, but it's worth a fiddle

http://www.refx.net/pro_Claw.htm?lang=eng

alternativly, Ive had great acid fun with the logic synth (es1, es2) and rcgaudio pentagon I. not 303 simulators, but capable of insane squigy madness once you get beyond the "this sounds like clingfilm" stage.