NetHack is a popular single player dungeon exploration game that runs on a wide variety of computer systems.
The Debug Assistant Library (DAL) is an assistant library for debugging, across platforms with uniform interface.
Space - A DirectX Game, is a DirectX, C, C++ based game that can be runned on any OS system. Needs DirectX in order to run effectively and cleanly. Peer-to-Peer, LAN, games as well as international gaming is available. Runs with 2+ players.
vMac is a free Open Source Macintosh emulator that is available for Linux/X-Windows, Win32, DOS, MacOS, and more.