Orx: Data-Driven Game Engine
Delphi Java - VRCalc++ OOSL (Script) and + (Binary Exec Distro)
A cross-platform port of Hexen II game.
Tools and Z80 engine for creating adventure games. (RUST)
RPG dice roll simulator for vintage Tandy TRS-80 100/102/200 laptops
Multi-platform game engine similar to old Megaman with a stage editor
Open Game Bootstrap
A generic 2D engine for any kind of game
The IRC's Talking Robot
A Simple Game Engine
Open Source Virtual World and MMOG Development Platform
Reference implementation for the Doom game engine.