Free software programmed in Tcl

aMSN was a free Windows Live Messenger clone. aMSN attempted to emulate the look and feel of Windows Live Messenger, and supported many of its features. It had been downloaded approximately 40 million times as of January 2011, making it the 21st most downloaded project on SourceForge.

The Visualization Toolkit (VTK) is a free software system for 3D computer graphics, image processing and scientific visualization.

Eggdrop is a popular IRC bot and the oldest that is still being maintained.

BRL-CAD is a constructive solid geometry (CSG) solid modeling computer-aided design (CAD) system. It includes an interactive geometry editor, ray tracing support for graphics rendering and geometric analysis, computer network distributed framebuffer support, scripting, image-processing and signal-processing tools. The entire package is distributed in source code and binary form.

MacPorts, formerly DarwinPorts, is a package manager for macOS and Darwin. It is an open-source software project that aims to simplify the installation of other open source software. It is similar in function to Fink and the BSD ports collections.

Expect is an extension to the Tcl scripting language written by Don Libes. The program automates interactions with programs that expose a text terminal interface. Expect, originally written in 1990 for the Unix platform, has since become available for Microsoft Windows and other systems.

image analysis and scientific visualization software

Tkabber is a GPL instant messaging client for the XMPP protocol which uses the Tk toolkit for the GUI. It runs on anything to which Tcl/Tk is ported: almost any X-based system (namely, any Linux and *BSD, Solaris, etc.), Microsoft Windows, and Mac OS X.

rattleCAD is a parametric 2D computer-aided design (CAD) software specific for bicycle design, in particular for design bicycle frame, developed by the Austrian cyclist and programmer Manfred Rosenberger since 2008. The application is written in the programming language Tcl using the Tk-based graphical user interface (GUI).

DejaGnu is a software framework for testing other programs. It has a main script called runtest that goes through a directory looking at configuration files and then runs some tests with given criteria. The purpose of the DejaGnu package is to provide a single front end for all tests. It is a part of the GNU Project and is licensed under the GPL. It is based on Expect, which is in turn based on Tcl. The current maintainers are Rob Savoye and Ben Elliston.

AOLserver is AOL's open source web server. AOLserver is multithreaded, Tcl-enabled, and used for large scale, dynamic web sites.