Manic Digger is an open-source implementation of block-building games as made famous by Minecraft. Manic Digger aims to be a generic platform ontop of which you can create your own game-types and scenerios. Through server-side scripting, any type of block-building game should be possible.
Simply rating it 1/10 and saying "STOLE MINECRAFT" is far from a review, it's idiocy, it comes from the sort of people that hate Saints Row for copying GTA 4, hang on a minute there aren't that many of those, why is that I hear you ask?! Well it's because Saints Row did it BETTER than GTA. Now I'm not saying that this is better than Minecraft at the moment or that it ever will be. What I am saying is that it should be judged on how good it is as an entity in the same way that both GTA 4 and Saints Row (2) are good games, one is just better than the other in the same way that minecraft is better than this (imo).
After spending some time both playing and developing this game I have to say I'm kinda addicted :)
Recently there's been a lot of changes both in gameplay as well as in graphics. Unfortunately these aren't officially released yet (have a look at the forum).
I recommend this game for everyone looking for a block-building game that's free and not resource-hungry. The game can be run on Windows, Linux and Mac (using Mono).
It's definitely worth a try.