I'm very late on this, but it's strange that nobody has mentioned the LispWorks IDE here, and it even has some GUI-Builder, which is portable across platforms (minus Mac OS X).
Also if you want a whole operating system for your Lisp programming undertaking, there still does exist Open Genera (which just runs on Dec Alphas (who is currently the owner of DEC ;-(), it's a complete OS written in Lisp and even has a C compiler, implemented in Lisp AFAIK, targeting the OS. It's very strange. You'll find a few things which we nowadays take for granted. E.g hyper referenced documents (but this was before the Web). It has a few "Lisp" dialects and the base is ZetaLisp, but Common Lisp works. All the tools and things can be introspected during runtime. It's a very strange feeling.
However, I just mentioned it. I guess nobody here will ever have touched OpenGenera....
