Steel Bank Common Lisp (SBCL) is an open source (free software) compiler and runtime system for ANSI Common Lisp. It provides an interactive environment including an integrated native compiler, a debugger, and many extensions.
SBCL runs on a number of POSIX platforms, and experimentally on Windows. See the download page for supported platforms, and getting started guide for additional help.
The most recent version is SBCL 1.0.22, released October 30, 2008 (release notes).
SBCL has a work in progress manual in the TeXInfo format. See the doc/manual directory in the source code for the current version in TeXInfo source.
Reporting a bug in SBCL should be done by sending e-mail to the sbcl-devel mailing list. The master record of known bugs is in the BUGS file, which is distributed with SBCL.