Macintosh Common Lisp 5.0b - despite its hefty price and what I consider to be the rather questionable way that recent subscribers were strung along with updates to classic versions for as long as possible until even the last of the free updates ran out and thus everyone has to pay the hefty prices, it's still one of the best Common Lisp implementations I know of.
Well, I'll refrain from the upgrade anyway and continue to rely on Open MCL - a free implementation of Common Lisp that is based on an older MCL version. What Gary Byers is doing there is really impressive and eventually there will be a GUI there too.
Found at VersionTracker.com - Mac OS X.