Free Lisp Compilers

Steel Bank Common Lisp
Steel Bank Common Lisp (SBCL) is a high performance Common Lisp compiler. It is open source / free software, with a permissive license. In addition to the compiler and runtime system for ANSI Common Lisp, it provides an interactive environment including a debugger, a statistical profiler, a code coverage tool, and many other extensions.

An implementation of Lisp for Mac OS 9. PowerLisp 2.02 is being released with complete source code (the entire product, including editor, can be built from scratch). This product is free for personal use

An interactive, Lisp-like language for .Net scripting and development, having deep .Net integration, sharing type system, GC and other runtime services etc., with transparent access to .Net w/o a FFI or wrappers.

The Free Poplog Portal
POPLOG is a free, open source, multi-language software development environment providing incremental compilers for a number of interactive programming languages: Pop-11, ML, Common Lisp and Prolog. Supported platforms appear to include Windows, Linux and Solaris (and possibly others)

CLISP includes a Common LISP interpreter and a compiler that support the ANSI Common LISP standard. CLISP runs on microcomputers (OS/2, Windows 95/98/NT/2000/XP, Amiga 500-4000, Acorn RISC PC) as well as on Unix workstations (GNU/Linux, BSD, SVR4, Sun4, DEC Alpha OSF, HP-UX, NeXTstep, SGI, AIX, Sun3 and others) and needs only 2 MB of RAM.

Lisp Studio v2.0
Common Lisp development system. Includes command-line and IDE Lisp interpreters. Supported platforms appear to include Win32 and .NET environment.

CMUCL is a free implementation of the Common Lisp programming language which runs on most major Unix platforms. It mainly conforms to the ANSI Common Lisp standard.