ProLisp An interpreter for a lexically scoped minimalistic Lisp of Lisp 1.5 type. It written in C++ and currently runs under Linux. [Open source]
Sapiens Software Star Sapphire Common LISP A free DOS-based Common LISP implementation with CLOS from Sapiens Software. Site includes a comprehensive reference manual for Common LISP.
Ufasoft LISP Common Lisp development system. Includes command-line and IDE Lisp interpreters. Possibility of creating EXE-files. [Free for non-commercial and education use]
Wade's Common Lisp: WCL Implements Common Lisp for Linux on x86 hardware, has large subset of Common Lisp as a shared library that can be linked with Lisp and C code to make efficient, small programs; suitable for embedding in other programs.
Xanalys LispWorks An integrated cross-platform development tool for ANSI Common Lisp. [Free and commercial versions]
XLISP-PLUS A version of Lisp based on the experimental XLisp.
XLisp-Stat Based on XLisp, but with built-in statistics support.