Dolphin Bassline is a free VST instrument plug-in for Windows PC.
This little monophonic synth is a proof-of-concept toy inspired by a well-known silverbox from the early 1980s. Empasis is on "inspired" as it isn't exactly an accurate model.
It has an oscillator, which you can ask to output either saw-waves or square-waves. The output of the oscillators goes into a variable frequency filter with optional resonance. Finally the output of the filter is multiplied by a slow, fixed-time envelope. Tweaking the knobs is suggested.
The oscillator will simply track the latest MIDI note-on message, so you'll probably want to sequence it. MIDI note-off messages matching the current note will gate the amplitude envelope. Overlapped notes won't retrigger the envelopes, and will cause the oscillator to slide to the new pitch. Notes with high velocity (about 101 and up) will trigger "accents" (these can be sliding notes too, or even half-way through the same note if you manage to convince your host not to send note-off in the middle), which will temporarily set the filter envelope timing to minimum and depending on the accent knob (and for slides also the remaining level of the filter envelope) boost volume and cutoff a bit.
Dolphin Bassline is available to download for free.