Legacy Common Lisp implementations
Commercial Common Lisp implementations that are no longer actively developed:

Symbolics Technology, Inc.
Genera (Symbolics Lisp Machines). Open Genera (Tru64 Unix)

Medley: MS-DOS, Unix

Star Sapphire Common Lisp
Free to download copyrighted unsupported shareware for MS-DOS

Gold Hill
Windows 3.1, Windows 95, Windows NT, OS/2

Corman Technologies
Corman Lisp: Windows. Trial versions. (open sources available)
PowerLisp: Mac OS 9. Free for non-commercial use.

Lucid Common Lisp
Now maintained as Liquid Common Lisp by LispWorks