Ready Lisp is the simplest way to get a full Common Lisp environment
running under OS X Leopard. Just download the disk image, and double-
click the application icon. You can even run it straight from the
disk image, if all you want to do is play around and try out Lisp.
Version 20080428 updates SBCL to version 1.0.16, SLIME to version
2008-04-24, and Aquamacs to 1.3b. It also disables 64-bit and
experimental threading support, as both have been known to have issues
on OS X.
http://www.newartisans.com/software/readylisp.html
Enjoy,
John