It is a small but complete implementation which features a very small bytecode interpreter, compilation via C, generation of standalone executables and DLLs, loadable binary files, CLOS, CLX, sockets, and more... The areas that need work are
There is a project page. ECL runs on at least Linux, FreeBSD, NetBSD, Windows (Using Mingw and Microsoft Visual C++), MacOS X/Darwin (x86 and PPC) and Solaris (x86 and Sparc). It supports threads on most operating systems.
I compiled it using mingw gcc under win32. It seems to need gcc at runtime to create fas files. threading also works under win32 making it the best free common lisp implementation available on that platform.