Minetest is a near-infinite-world, open-source block sandbox game and game engine inspired by Minecraft. With lots of mods available, several different in-house map generators, and a near-infinite playing area, you are in control of your own world.
Open Souce,
Minimal sys Requirements.
Largely popular with multiple OS's,
Updated almost every day!
Cons
Old Graphics
Summary
Great game :)
seems to work well
ChoiceVoice
Pros
-open source
-free
-very similar to minecraft
-portable if you want
-very lite.
Cons
-i'm trying it on an obsolete, ancient laptop and can't read the font very well. but it's impressive it runs on this old beast.
-not sure how to configure it (could use some help files or something - i know there's something in wiki about this though).
Summary
i haven't really dug down too deep into this virtual world. but it seems good. well done open source boys.