Computer Algebra System written in Common Lisp
Maxima is a computer algebra system comparable to commercial systems like Mathematica and Maple. It emphasizes symbolic mathematical computation: algebra, trigonometry, calculus, and much more. For example, Maxima solves x^2-r*x-s^2-r*s=0 giving the symbolic results [x=r+s, x=-s]. Maxima can calculate with exact integers and fractions, native floating-point and high-precision big floats. Maxima has user-friendly front-ends, an on-line manual, plotting commands, and numerical libraries. Users can write programs in its native programming language, and many have contributed useful packages in a variety of areas over the decades. Maxima is GPL-licensed and largely written in Common Lisp. Executables can be downloaded for Windows, Mac, Linux, and Android; source code is also available. An active community maintains and extends the system. Maxima is widely used: annual direct downloads exceed 300,000. Many other users receive it through secondary distribution.
A universal instant messaging (IM) program
See https://pidgin.im/about/ for more information. Pidgin is an instant messaging program which lets you log in to accounts on multiple chat networks simultaneously. It runs on Windows, Linux, and other UNIX operating systems. Pidgin is compatible with the following chat networks out of the box: AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, MXit, Novell GroupWise Messenger, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr. It is written in C and makes heavy use of GLib and GTK+. Finch is a command line instant messaging program. It also lets you log in to accounts on multiple chat network simultaneously, and it is compatible with the same chat networks as Pidgin. It is written in C and makes heavy use of GLib and ncurses.
Chess Database and Toolkit program
"Shane's Chess Information Database" is a huge chess toolkit with extensive database, analysis and chess-playing features. Scid vs. PC is a usability and bug-fix fork of Scid. It has extensive interface fixes and improvements, and is fully compatible with Scid's .si4 databases. It's new features include a rewitten Gamelist, a Computer Tournament, and FICS, Tree and Book improvements.
AudeLA is a TCL extension aimed at providing amateur astronomers with image processing, telescope controling, ccd camera driving, and various astronomical algorithms.