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.
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.