LispWorks for Macintosh [http://www.lispworks.com/products/lwm.html] is a commercial implementation of ANSI Common Lisp [http://www.lisp.org/]. A Free Personal Edition is available [http://www.lispworks.com/downloads/].
LispWorks has a greater than 15 year tradition on the Unix and Windows platforms. Since July 2003 a port of LispWorks to Mac OS X is available.
One of the outstanding features of LispWorks is its cross-platform GUI lib. It makes it possible that LispWorks has the same extensive integrated development environment on Windows, GTK+, X11/Motif and Mac OS X / Cocoa.
Lispworks screenshot gallery: http://lemonodor.com/lwm-4.3-os-x/1.shtml
On the Mac, the whole development environment has been ported to Cocoa - which is pretty cool:
On the Mac, the X11/Motif version is also included: http://lemonodor.com/lwm-4.3-os-x/thumb/lwm1.jpg.
LispWorks has a fast incremental compiler and compiles directly to native machine code.
Like any implementation of ANSI Common Lisp, LispWorks supports CLOS, the Common Lisp Object System. CLOS provides the Lisp developer with a rich facility for object-oriented programming (objects, classes, slots, generic functions, methods, meta classes, multiple inheritance, multi-dispatch, …).
LispWorks has a lot of additional features like threads, meta-object protocol, database access, Prolog, Corba support, etc.
With the Professional and Enterprise editions of LispWorks one can write and deliver royalty-free Common Lisp applications.
LispWorks has a documented Cocoa interface ([http://www.lispworks.com/documentation/lw60/OBJC/html/objc.htm]).
See also: AllegroCL, OpenMCL