http://www.tokyodawn.net/tdr-feedback-compressor-2/ This is one of the few FREE plugins that is not only good, but can stand with the best of anything out there. Works great, and yes, it's 64-bit.
Free / cheap plugins are getting so good that it's hard to justify spending real money on others. Good times!
Addendum: after playing with this for a little bit, it's already earned spots on my default 2bus and drum bus. Fantastic compressor, especially for bus / mastering duties. Did I mention it's free? Amazing.
Cheers,
Eddie