Me and a few folks are working on a game that's loosely inspired by Minecraft (as well as, like, a dozen other things). No, it won't be open source, but our plan does include a full suite of developer tools. Current plans are to use C# and .Net, perhaps Unity as well if we can find a good voxel-based engine that doesn't cost $3,000 (Yeah, I'm lookin at you, TerrainEngine, you beautiful, horridly expensive thing). It'll also look a lot prettier than Minecraft, something like the video below. Yes, we fully intend to actually sell the game for cold, hard cash. Probably thru Steam. Its still largely in the planning and "choosing a bloody engine" phases, but we're quite serious about this.