Common Lisp
Common Lisp is a standardized Lisp dialect standardized by ANSI as ANSI INCITS 226-1994 (R2004) in 1994. A hyperlinked version of the standard is available in the form of the HyperSpec.

CLiki is a wiki site providing a wide range of information about Common Lisp and Free Software Common Lisp projects. CLiki contains an up-to-date list of free/libre/open-source software Common Lisp implementations.

Dan Weinreb's Common Lisp implementation survey includes a lot of detailed information (recent as of Feb. 2010) about the different implementations.

Below is a list of Common Lisp implementation from commercial vendors:

Commercial Vendors

Allegro Common Lisp from Franz, Inc.
Platforms: Mac OS X, Microsoft Windows XP/Server/Vista/7, Solaris, AIX, FreeBSD, Linux

Platforms: Mac OS X, Microsoft Windows XP/Server/Vista/7, FreeBSD, Solaris, HP-UX, Linux

Scieneer Pty Ltd

Free/Libre/Open-Source Software Common Lisp Implementations

The canonical list of FLOSS Common Lisp implementations is kept on CLiki.

Legacy Commercial Implementations

See Legacy Common Lisp implementations