Green Oak Crystal is highly considered by many, especially considering it is a free app.
If you’ve used Crystal, leave a comment and let us know what you think of it!
Crystal is a semi-modular software synthesizer featuring both subtractive synthesis and frequency modulation (FM) synthesis. It offers over 90 parameters which may be modulated, multi-stage envelopes with graphical editors, extensive tempo sync of envelopes/lforates/delay times, built-in effects for chorus/flanging/comb filtering/echoes, and a band splitter for effects processing by frequency band.
Crystal has wave sequencing, granular synthesis, program morphing, MIDI learn, and imports soundfonts.
With version 2.5.0, Crystal is now available as a 64 bit plugin on both Windows and OSX. The OS X installer installs both Audio Unit and VST plugins which are capable of 32 or 64 bit. The Windows download includes both 32 and 64 VST versions, so you can put the appropriate one in your plugins folder.