d-lusion has released two virtual instruments as free downloads, Drumstation and Rubberduck.
Drumstation is a free, fully functional “drum computer” for your PC. Its concept is based on the legendary cult synthesizer TR 909. Rubberduck is a free, fully functional acid- and techno-synthesizer for your PC, based on the legendary cult synthesizer TB 303. Drumstation combines cool old drum machine features with cutting-edge software-synthesis technology. Drumstation features 8 channels of drums (either samples or synthesized drum sounds), programmable via an easy-to-use step sequencer, with effects (realtime reverb, delay, flanger, filter, distortion) for each channel.
Rubberduck’s synthesis model consists of an oscillator wave with a resonant filter sweep applied to it combined with an efficient hardware sequencer design which resolves in the typical twisted, screaming, bubbling bass sound that is often used in Acid House, Goa Trance, or Techno productions. Rubberduck features realtime digital resonant dynamic filters, frequency and volume envelope (DCA, DCF), 4 basic waveforms, dual oscillator sound generation (starting with version 2.0), a 224 note sequencer, a drum section where samples could be synchroneous played back, an effect unit (delay/feedback/distortion) and is fully Windows 95 compatible.
Both are available as free downloads from the d-lusion site.