Subatomic labs has introduced the Mantis307 - a bass soft synth inspired by the TB-303 (duh..), Juno 60, OSCar etc.

"Features killer unison and distortion, as well as constant time slide (portamento done right!). In addition, 2 oscillators + subosc per voice, sync, mono and polyphonic modes, keyboard tracking, and 3 envelope curve types. (and much much more!). And with a super low CPU footprint that lets you layer many mantis sounds without breaking a sweat.

Mantis307 was created for raw acidic sound design! ::: Mantis really shines when used like a tb303, as a bassline monosynth. To get you started, there are several teebee presets to get the aciiid juices flowing. But don't be fooled, Mantis's range of sounds include much much more than just 303 and basses."

Some sound clips, all non-drum sounds by Mantis307:

TBDemo

AcidBass

Check out Subatomic Labs site for more info and sound clips!

The Mantis307 is free, but for $20 you get a registration code that gets rid of a nag screen.