Minetest is an open source voxel game engine. It is free to download and play, does not require java, and runs faster on older hardware than some other voxel games out there coughMINECRAFTcough
The current Mesozoic Minetest build is a Minetest 5.0.1 server running MineClone 2
Minetest runs on Windows, Mac, *nix, and even mobile (YMMV), to get started, go download the version for your platform (5.1.0 will run fine on our 5.0.1 server), and use the connection information below