The Newtonator is an LV2 soft synth that uses a unique algorithm based on simple ideas of velocity and acceleration to produce some unpredictable sounds.
unfa has published an extensive video tutorial on how to synthesize a kick drum, a snare drum, a hihat and a crash cymbal using the softsynth ZynAddSubFX.
AMSynth version 1.7.0 has just been released. AMSynth is an open source realtime software synthesizer for Linux. Its operation is similar to analog Moog Minimoog and Roland Juno-60, which are considered classic synthesizers from the 1970s. It is available in various formats, including LV2 and DSSI plugins as well as JACK/ALSA standalone clients.
Helm is a free, cross-platform, polyphonic synthesizer that runs on GNU/Linux, Mac, and Windows as a standalone program and as a LV2/VST/AU/AAX plugin. It sounds absolutely enormous, has a very distinct personality, and is generally very fun to use.
The release is in beta, but given a powerful enough machine to handle the very busy UI, it is remarkably stable.