Darwin
The open source part of MacOS X. Darwin includes the UNIX kernel and BSD-based user-land command line utilities.

As Darwin is the bottom-most layer of MacOS X, any Common Lisp implementation that works there will work with Darwin, with the exception of any that require Apple's proprietary Aqua GUI, of which there are none. LispWorks, for instance, can use Aqua, but will work without it, from the command line.

Darwin even works on the Intel x86 (PC)!

There are various projects to extend the Darwin experience, including OpenDarwin, which houses many sub-projects, such as DarwinPorts which include a few pre-packaged Common Lisp implementations.

Darwin can be downloaded from Apple or from OpenDarwin.Org.