One of the chief difficulties in learning Common Lisp development is finding a useful IDE. Here are a list of choices which may work at present (2013).
The canonical free IDE is undoubtably emacs, but it is difficult for newbies. The author has had good experience with messing around on LispWorks.
Portacle - a portable and multiplatform Common Lisp development environment. It packages SBCL, Emacs25, Slime, Quicklisp and Git altogether. No installation needed.
vim/SLIME - Maintained.
McClide (OSX) - Maintained.
Ufasoft (Windows) - Unmaintained.
Atom-SLIME - Maintained. Integrates SLIME with Atom! This package allows you to interactively develop Common Lisp code, helping turn Atom into a full-featured Lisp IDE.