Robot Planet has released a Mac OS X VST version of the free "BattleComp Vintage" compressor plug-in.
BattleComp Vintage is a punchy, yet smooth compressor with an EQ and a high-pass filter on its internal sidechain and tube emulation + soft-clipping circuit in the output stage to keep peaks under control. The high-pass filter makes it easy to avoid pumping effects when there's a lot of bass - something many compressors struggle with.
This compressor is suitable for both master signal compression and per-instrument compression.
BattleComp Vintage is free and is available for Mac OS X and Windows as a VST effect plug-in.