A free, multiplayer game that focuses on fast-paced melee, subterranean exploration and building, OpenKlonk is the successor of the shareware series entitled Clonk; a substantial explanation of its checkered history here at the game's wikipedia. Released under the ISC license, OpenClonk has a lot of features that would appeal both to the budding developer and the casual gamer. While the visuals can seem somewhat rough, OpenClonk boasts smooth controls and relatively intuitive gameplay. There's something ridiculously satisfying about catapulting yourself through the caverns with a grappling hook.
http://www.openclonk.org/