from KVR-vst.com: Audio Units is an alternative, OS X only, plugin format to VST and is the only plugin format supported by Emagic's Logic on OS X. The Crystal soft synthesizer, previously only available for VST, is now available as one of the first plugins in this new format. Crystal, one of the most popular software synthesizers due to its inspiring sonic capabilities, along with the added bonus that it's free, may now be used by Logic 6 (version 6 of Logic is required), and other AU host users on OS X. Download it here: http://www.greenoak.com/crystal