Not sure if this has been around these forums or not, but I stumbled accross a little device from Audiorealism and let me just say from first hand experience, this thing sounds amazing, very very real, of course some styles of sound can be a little VSTi'sh, but there are some very convincing demos.
The site also provides A/B's against the real TB303 and the sound is spot on, I have owned a real tb303 for ide say the last 7 years so I know the sound pretty well I think, and I just purchased this item off the demos alone, ill post a demo once its in my hands for a bit, here is a link to the site..
Site: http://www.audiorealism.se/index.htm
A convincing demo: http://www.audiorealism.se/demos/fmu-abl-demo1.mp3
Have a listen and you be the judge, there also some artists who are known in the electronic scene for being large users of tb303s and they post in the forums, a nice touch

Just thought I would share incase any one wanted a nice little device at a modest price, and best of all, it has a step sequencer built into the unit and its VSTi capable.
Cheers!