Crystal for Mac

From Green Oak:

Crystal is a semi-modular software synthesizer featuring both subtractive synthesis and frequency modulation (FM) synthesis. It offers abundant modulation control with 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. Not only that, Crystal has wave sequencing, granular synthesis, program morphing, MIDI learn, and imports soundfonts. The musical applications for Crystal are literally boundless.

Crystal is a plugin, which means it runs within a host, such as Cubase, Orion, Logic, Fruity Loops, etc.