ReBirth RB-338 is FREE!

Propellerhead's Rebirth is free now (it has been since 2005). The only downer is that the website that was hosting it seems to have gone, though there is a torrent, it's 220 MB.
EDIT: The Rebirth Museum is back online.

Hobnox : Everybody Needs a 303

If you are bored and want to play with two 303's, a 909 and some guitar pedals then check out Hobnox Audiotools. It's all online which is a plus, but the downside is that there is no record button. But this is easily fixed by pressing record in your DAW or sound recorder etc. That being said though, in version 2.0 (coming soon) you will be able to save and load your work. Though to what extent remains to be seen.