ReBirth 338 is a stand alone program that emulates the TB303 and TR808/909 machines. This software sequencer has an astoundingly accurate sound, and a very unique look. There is a complete pattern based sequencer included that is used to build up songs, but the real-time control of the synth parameters as part of the song is really where the fun starts.
ReBirth can also send all its outputs to full version of Cubase VST's mixer for further processing.
The Demo has all the features of the commercial version, but has no mods. The Demo works for 15 minutes and can't save songs or export audio to aiff/wav.