A few suggestions that would be cool with source available.

- UFO: Alien Invasion - an original 3D game in the spirit of the old UFO/X-COM games
- UFO2000 - an open source game engine for the old X-COM: Ufo Defense / UFO: Enemy Unknown game
- FreeDink - a hack'n'slash adventure released for free with source
- Chaos Strikes Back - an engine for the Dungeon Master games
- Dune 2 - a game engine for Dune 2