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. Release is in beta so there are outstanding bugs. Please send any bugs found to matthewtytel@gmail.com

The main goals of Helm are ease of use and software freedom. A lot of development has been focused on creating nice graphical widgets so you can see what's going on. It's licensed under GPLv3 so it's free to use/distribute/change and all that.

-- Here's a feature overview --
GLOBAL
32 voice polyphony
Arpeggiator
Cross-platform patch loading/saving
Oscilloscope
Portamento/Legato

OSCILLATORS
2 main oscillators with 11 anti-aliased waveforms, cross modulation, unison/harmonize
A sub oscillator with 'shuffle' wave shaping and a noise oscillator
Oscillator feedback for changing timbre

MODULATION
Routable modulation and real time visual feedback so you can see what controls are modulated and by how much
Aftertouch/Velocity/Note/Pitch Wheel/Mod Wheel modulation sources
2 mono and 1 poly LFO with live visual feedback
2 Envelopes
Step Sequencer with smoothing

FILTER
Filter with low-pass, high-pass, band-pass, high-shelf, low-shelf, band-shelf and all-pass settings
Formant Filter with x/y pad control

EFFECTS
Stutter
Delay
Reverb