ZynAddSubFX is a free, open source (GNU GPL) powerful, realtime, multi-timbral software synthesizer for Linux and Windows. An alpha release of the VST version has just been released (v2.0.0 pre 2).

Features:

• Realtime, Polyphonic, Multi-timbral synthesizer.
• Microtonal capabilities with any scale, any number of notes per octave (up to 127) and keymapping.
• 3 synthesizer engines:
  1. ADsynth" or "ADnote" is a complex engine which makes sounds by adding a number of voices. Each one has filters, envelopes, LFOs, morphing, modulation (Ring Modulation, Phase Modulation, the modulators can have any shape), resonance, etc. Each voice includes a very powerful waveform generator with up to 128 sine/nonsine harmonics. This engine includes antialiasing.
  2. "SUBsynth" or "SUBnote" is a simpler engine which allows you to make sounds by subtraction of harmonics of sounds from white noise.
  3. "PADsynth" or "PADnote") makes beautiful pads and other instruments.
• Instruments can be organized in kits, which allows you to make drum kits or layered (mixed) instruments; this makes it possible to use more than one instrument for a single part. It is possible to choose what items from the kit should be processed by the Part's effects.
• Effects: Reverb, Echo, Chorus/Flange, Phaser, AlienWah (a kind of vocal morpher), Distortion (Waveshaping), EQ, DynamicFilter.
• The effects can behave as System Effects or as Insert Effects; this allows you to use many effects at the same time. You can put any insert effect on any part (or more insert effects to same parts, if you like) or you can put the insert effect to Master output. Also, you can have instrument's effect (that is saved with the instrument).
• Filters up to 60dB/octave (or more); filters can be analogue (LPF,HPF, BPF,etc.), formant (advanced formant filters) and state variable (SVF).
• Envelopes can have ADSR (or ASR, etc.) modes or can be free modes (with any shape).
• Many settings have a "randomness" option; this produces a very pleasant effect (like analogue synths or natural instruments that can't produces exactly same sound twice).
• Intuitive and easy to use User Interface.
• MIDI support (supports many controllers) and (if you want) JACK support.
• Virtual Keyboard.
• It is free, licensed under version 2 of the GNU General Public License.