There's a new free synth from developer Matt Tytel.
Helm is a cross-platform, polyphonic synth plug-in that runs on GNU/Linux, Mac, and Windows as a standalone program and as a LV2/VST/AU/AAX plugin, best of all it's free.
- 32 voice polyphony
- Interactive visual interface
- Powerful modulation system with live visual feedback
- Dual oscillators with cross modulation and up to 15 oscillators each
- Unison and Harmony mode for oscillators
- Oscillator feedback and saturation for waveshaping
- 12 different waveforms
- 7 filter types with keytracking
- 2 monophonic and 1 polyphonic LFO
- Step sequencer
- Lots of modulation sources including polyphonic aftertouch
- Simple arpeggiator
- Effects: Formant filter, stutter, delay