Free 32/64/16-bit multi-platform Pascal and Object Pascal compiler
A 32/64/16-bit Pascal compiler for Win32/64/CE, Linux, Mac OS X/iOS, FreeBSD, OS/2, Game Boy Advance, Nintendo NDS and DOS; semantically compatible with Delphi, Borland Pascal and Mac Pascal (partially) with extra features, e.g. operator overloading.
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.
Hammer of Thyrion (uHexen2) is a cross-platform port of Raven Software's Hexen II source. It is based on an older linux port, Anvil of Thyrion. HoT includes countless bug fixes, improved music, sound and video modes, opengl improvements, support for many operating systems and architectures, and documentation among many others.
OpenTTD is an open source clone of the Microprose game "Transport Tycoon Deluxe". For more recent versions visit our website: http://www.openttd.org
Old style soccer game with many exciting features
Eat the whistle GPL is a football simulation game written in C that works on many platforms (win32/linux/macosx/amigaos/morphos), it uses the SDL framework. It has a multiplayer internet playing option in alpha stage.
Scalar is an addictive puzzle game. The goal is to assemble images from pieces that are shuffled randomly. The game is based on SDL and works on Windows, Linux and MorphOS so far.
Schism Tracker is a music editor that aims to match the look and feel of Impulse Tracker as closely as possible. This SourceForge project page is more or less abandoned -- current development is ongoing at schismtracker.org.