>>21
Yep. Chicken Scheme is probably my personal favorite, but SBCL and the libraries that work with it lead in terms of activity, making them the most practical thing to turn to for a lot of complicated applications or real-world deployments. The last time I looked, there were way less libraries.
Chicken is great and is a rare exception to the problems of the Lisp community, IMO. Installing libraries is nice (it's fine in CL with Quicklisp, don't get me wrong) and still seems to be going strong.