This is a Java applet you can reach at : http://kfn.ksp.or.jp/~munehiro/tb303/jb303.html
The sound it produces is a bit weak, but fun to use !
If you're a proud owner of the Roland MC-202, the this program will catch your attention, listen : it converts any monophonic midi file to a MC-202 sequence that you can easily dump to the "tape in" socket of the MC-202.
The program has a nice feature : a 303-ish mode ! Yep, it could easily emulate 303 slide on the MC-202, a big bonus if you to recreate that Josh Wink acid bassline on the same instrument he used !!!
Get 202 Hack at http://www.welch.jhu.edu/~geh/defective.html