Common Lisp

Do any of you use common lisp? If so, which implementation do you suggest and why?

I use SBCL, works fine on both *NIX and Windows.
Source is public domain and easy to inspect, of course for development environment, I'm using Emacs and SLIME.

Clozure(not to be confused with Clojure), or CCL, seems like another good free alternative. Allegro or LispWorks for commercial ones.