Free software programmed in Lisp

Apel (emacs)
In computer programming, apel (the initialism represents "A Portable Emacs Library") provides support for writing portable code in Emacs Lisp.

Axiom (computer algebra system)
Axiom is a free general purpose computer algebra system.

Climacs
Climacs is an open source text editor written in Common Lisp that is similar to GNU Emacs and is released under the GNU LGPL software license.

CLPython
CLPython is an implementation of the Python programming language written in Common Lisp.

Emacs
Emacs is a family of text editors, characterized by their extensibility.

Emacs/W3
Emacs/W3 is a text-based web browser for the GNU Emacs text editor, written primarily by William M. Perry and entirely in GNU Emacs Lisp.

Emacspeak
Emacspeak is a free computer application, a speech interface and an audio desktop (as opposed to a screen reader) employing Emacs, which is written in C, Emacs Lisp and Tcl and developed princip...

Gnus
Gnus is a message reader running under GNU Emacs and XEmacs.

LKB
Linguistic Knowledge Builder (LKB) is a free and open source grammar engineering environment for creating grammars and lexicons of natural languages.

Maxima (software)
Maxima is a computer algebra system based on a 1982 version of Macsyma.

Prototype Verification System
The Prototype Verification System (PVS) is a specification language integrated with support tools and an automated theorem prover.

Reduce (computer algebra system)
Reduce is a general-purpose computer algebra system geared towards applications in physics.

Sawfish (window manager)
Sawfish is an extensible window manager for the X Window System.

SLIME
SLIME, the Superior Lisp Interaction Mode for Emacs, is an Emacs mode for developing Common Lisp applications.