I have heard of MineCraft before--its come up a lot must be worth investigating.
If you've ever played Elite Beat Agents for the Nintendo DS, you'd understand the basic idea of osu!, though it also helps if you have passing familiarity with Stepmania.
Basically it's a rhythm game that uses user-generated note-charts. Gameplay is as simple as clicking on a note when the circle outside it meets the rest of the circle, and dragging notes along tracks when you get to track notes. I think there may also be spinners, but f*** spinners.
Minecraft on the other hand is something I'll admit to never actually playing, but from what I understand it's an open-world RPG-like game in which the world is practically Lego to you, capable of being destroyed for its material and reconstructed somewhere else. You can go about playing a proper world, or you can just turn your world into a sandbox for you to build whatever you want. It supports online multiplayer, and there are countless servers out there devoted to specific building projects. --- http://i39.tinypic.com/xnar69.gif