Bitfighter is a game that feels a little like Asteroids on steroids. It's a multiplayer space-based combat and strategy game, featuring customized ships, multiple game modes, cool retro graphics, and smooth network play. It also includes a full featured level editor, Lua scriptable bots, and other cool features. It has a very active development team, so the game keeps improving.
Arx Libertatis is a cross-platform, open source port of Arx Fatalis, a 2002 first-person role-playing game developed by Arkane Studios. Arx Fatalis features crafting, melee and ranged combat, and a unique casting system where the player draws runes in real time to effect the desired spell. The Arx Libertatis source code is based on the publicly released Arx Fatalis sources. This does however not include the game data, so you need to obtain a copy of the original Arx Fatalis or its demo.
Humm and Strumm is a 3D adventure game in which two players must collaboratively solve puzzles and fight enemies in order to stop the evil Dr. Geoff from taking over the world. Humm and Strumm is set in a psychedelic fairytale world, with lands such as a giant cupcake and a musical forest. The main characters, Humm and Strumm, are armed with yo-yos for weapons. Humm and Strumm has two modes of play: Adventure Mode, in which two players fight Dr. Geoff, and Tag Mode, in which up to eight players play a game of tag using their characters' weapons.