Lispworks has a free "Personal Edition". It's good enough to get through something like Practical Common Lisp and get a feeling for writing lisp, however since

Lispworks has a free "Personal Edition". It's good enough to get through something like Practical Common Lisp and get a feeling for writing lisp, however since it uses an old version of ASDF, it's a bit difficult to use the "modern" quicklisp ecosystem.