Clozure CL (formerly known as OpenMCL) is a free Common Lisp
implementation. Features include:

  * A fast, precise, compacting, generational garbage collector
    written in hand-optimized C. The sizes of the generations are
    fully configurable.

  * Full native OS threads on all platforms. The API includes support
    for shared memory, locking, and blocking for OS operations such
    as I/O.

  * Full Unicode support.

  * Excellent debugging facilities. The names of all local variables
    are available in a backtrace.

  * A complete, mature foreign function interface.

  * Many extensions including: files mapped to Common Lisp vectors
    for fast file I/O; thread local hash tables and streams to
    eliminate locking overhead; cons hashing support.

WWW: http://www.clozure.com/clozurecl.html