I heard of Minetest a while back, it looked pretty alpha. Looking at it now it looks really good, I'll have to try it. I'd love a game that can recreate the Minecraft experience but be FOSS. If it's a pretty similar clone of Minecraft, I'd be happy.
Edit: Hmm, it is definitely missing a lot compared to Minecraft, as a game on it's own it seems like it'd be great. But I cannot help but compare it to Minecraft and unfortunately it just does not seem to be there yet. Animations are not similar, it's missing a ton of the content Minecraft currently has. It feels like an early version of Minecraft, which is understandable but I don't know if I can ever say I'd play this when Minecraft exists. Which is a shame because I want to like it, but eh. It needs work.