A partition and disk imaging/cloning program
Clonezilla is a partition and disk imaging/cloning program similar to True Image®. It saves and restores only used blocks in hard drive. Two types of Clonezilla are available, Clonezilla live and Clonezilla SE (Server Edition).
A planetarium software that shows a 3D simulation of the night sky
Stellarium renders 3D photo-realistic skies in real time with OpenGL. It displays stars, constellations, planets, nebulae and others things like ground, landscape, atmosphere, etc.
Psi IM — Qt-based XMPP/Jabber client
Psi aims to be a feature-rich Jabber/XMPP client for novice and experienced Jabber users. There are builds available for MS Windows, GNU/Linux and macOS.
Run Windows applications on Linux, BSD, Solaris, and Mac OS X
Wine is an Open Source implementation of the Windows API on top of X and Unix. Wine provides both a development toolkit for porting Windows sources to Unix and a program loader, allowing many unmodified Windows binaries to run on x86-based Unixes.
Rapid applications development tool and libraries for FPC
The Lazarus IDE is a stable and feature rich visual programming environment for the FreePascal Compiler. It supports the creation of self-standing graphical and console applications and runs on Linux, FreeBSD, MacOSX and Windows.
A cross-platform GUI library
wxWidgets is a C++ library that lets developers create applications for Windows, Mac OS X, Linux and other platforms with a single code base. It has popular language bindings for Python, Perl, Ruby and many other languages, and unlike other cross-platform toolkits, wxWidgets gives applications a truly native look and feel because it uses the platform's native API rather than emulating the GUI. It's also extensive, free, open-source and mature.
A premier mind-mapping software written in Java
A mind mapper, and at the same time an easy-to-operate hierarchical editor with strong emphasis on folding. These two are not really two different things, just two different descriptions of a single application. Often used for knowledge and content management.
Cross platform GUI for Maxima
wxMaxima is a cross-platform graphical front-end for the computer algebra system Maxima based on wxWidgets. It provides nice display of mathematical output and easy access to Maxima functions through menus and dialogs.
Xfce is an easy-to-use and easy-to-configure desktop environment using the popular GTK+ toolkit.
Another version of the simple action game modeled after the lightcycle sequence of the movie Tron.
Free flexible open source todo manager featuring hierarchical tasks
Task Coach - Your friendly task manager. Task Coach is a free open source todo manager. It grew out of frustration about other programs not handling composite tasks well. In addition to flexible composite tasks, Task Coach has grown to include prerequisites, prioritizing, effort tracking, category tags, budgets, notes, and many other features. However, users are not forced to use all these features; Task Coach can be as simple or complex as you need it to be. Task Coach is available for Windows, Mac OS X, and GNU/Linux; and there is a companion iOS app.
iTALC was a free classroom management software and has been replaced by Veyon: http://veyon.io
DjVu is a web-centric format for distributing documents and images. DjVu was created at AT&T Labs-Research and later sold to LizardTech Inc. DjVuLibre is a GPL implementation of DjVu maintained by the original inventors of DjVu.
Low-latency, high quality voice chat for gamers
Mumble is an open source, low-latency, high quality voice chat software primarily intended for use while gaming. It includes game linking, so voice from other players comes from the direction of their characters, and has echo cancellation so the sound from your loudspeakers won't be audible to other players.
Versatile Commodore Emulator
VICE is an emulator collection which emulates the C64, the C64-DTV, the C128, the VIC20, practically all PET models, the PLUS4 and the CBM-II (aka C610). It runs on Unix, MS-DOS, Win32, OS/2, Acorn RISC OS, BeOS, QNX 6.x, Amiga, GP2X or Mac OS X machines.
An award-winning drawing program for children ages 3 to 12
Tux Paint is a free, award-winning drawing program for children ages 3 to 12. It combines an easy-to-use interface, fun sound effects, and an encouraging cartoon mascot who guides children as they use the program. Kids are presented with a blank canvas and a variety of drawing tools to help them be creative. Along with paintbrush, shapes and text, Tux Paint includes a "stamp" feature to add pre-drawn or photographic imagery to pictures, and a set of "magic tools" that provide filter effects (like blur, tint and waves) and interesting drawing tools (like train tracks, bubbles and grass). Tux Paint includes a collection of "starter" images, both coloring-book style and photo-realistic, and a large collections of stamps are available as a separate download. Additional content can be added by parents and teachers, or downloaded from 3rd parties. Tux Paint is available for Linux, BSD, Windows, Mac OS X and Haiku operating systems.
A free multi-track audio editor and recorder
Audacity is a free, easy-to-use, multi-track audio editor and recorder for Windows, Mac OS X, GNU/Linux and other operating systems. The interface is translated into many languages. The version currently hosted here is 2.1.0 (March 2015). More recent versions than this are available from http://www.audacityteam.org/download/ . Audacity is free software, developed by a group of volunteers and distributed under the GNU General Public License (GPL). Programs like Audacity are also called open source software, because their source code is available for anyone to study or use. There are thousands of other free and open source programs, including the Firefox web browser, the LibreOffice or Apache OpenOffice office suites and entire Linux-based operating systems such as Ubuntu
gPhoto is a program and library framework that lets users download pictures from their digital cameras. The libgphoto2 library gives you access to hundreds of models of digital cameras on several platforms. GIT repo and Issue tracker lives at https://github.com/gphoto/ these days.
A Graphical Developement Environment based on a Basic Interpreter with object extensions.
Colonize the New World.
FreeCol is an open version of Colonization. It is a Civilization-like game in which the player has to conquer the new world.
This is one of the most popular Doom source ports. It is available for DOS, Windows, Linux and Mac OS X, and should build on any POSIX platform which supports SDL. It enhances Doom with things like TCP/IP networking, OpenGL renderer, Heretic support and much more.
Java applications to control a model railroad layout.
Java interfaces and sample implementations for controlling a model railroad layout from a personal computer. JMRI is intended as a jumping-off point for hobbyists to build their own layout controls. Includes the DecoderPro and PanelPro applications.
Panorama photo stitcher
With Hugin you can assemble a mosaic of photographs into a complete immersive panorama, stitch any series of overlapping pictures and much more.
OpenSource OpenGL Multiplayer Multiplatform Battle Zone capture the Flag. 3D first person tank game. Project has moved to Github: https://github.com/BZFlag-Dev/bzflag
"Tux, of Math Command" is a math drill game starring Tux, the Linux Penguin. Lessons are included from simple number typing through addition, subtraction, multiplication, and division of positive and negative numbers. It is intended for kids ~4-10.