Free, very quick, easy to use and rock solid. A top quality application.
I would recommend against using this Common Lisp implementation. No releases for going on five years, unlike Clozure Common Lisp (CCL) and Steel Bank Common Lisp (SBCL), which are much more actively maintained. If something goes wrong with one of those, you have a much better chance of seeing a fix.
It wipes out your PATH variable. Backup before using this.