"Ace of Spades is a free multiplayer game with voxel graphics and destructable terrain. The gameplay is capture-the-flag style."
Official Site
Download Link for the game
Download link for the server browser (makes joining games MUCH easier)
Orbitoid's nice zip with both installers in it
This game can be run under windows or linux (using wine(I've tested!)). I have had heaps of fun so far in it and it is super awesome. Looks "similar" to minecraft in its blocky appearance.
~~~NOTE~~~ you dont "need" the server browser but it makes it easy to change your name (you start off with the name deuce) and your in game resolution.
Also be sure not to press escape when in game as it quits without any warning.
Once you have the game and server browser installed find the "Respawn Australia" server in the list and join in the fun
The map i started with is "Caves0" which has a huge mountian in the center of the map with heaps of caves in the middle.