Ready Lisp is a binding together of several popular Common Lisp packages especially for Mac OS X, including: Aquamacs, SBCL and SLIME. Once downloaded, you'll have a single application bundle which you can double-click -- and find yourself in a fully configured Common Lisp REPL.
It's ideal for OS X users who want to try out the beauty of Common Lisp with a minimum of hassle. It could also be used by teachers to give their Mac students a free, complete Common Lisp environment to take home with them.