Minecraft is ugly, simple to the point of being stupid, and horribly entertaining & addictive.
Most of us have heard the term "sandbox environment" before, but for those of you who haven't, it's essentially just an open-world that allows you to shape it freely. Minecraft has it's own little spin on this concept where, in the current live alpha mode, you start with nothing and have to figure out how to first create tools from the world around you. For example: you'll need to collect logs from trees, shape them into boards and place them in a square shape in the crafting area to create a workbench. This then allows you to create tools, weapons, armor, staircases, ladders, food stuffs and countless other combinations. All of these items are used to dig, build, and defend yourself in your own personal little block-world.
Minecraft has already had an impressive community formed around it with people creating additional programs for modifying, mapping, and adding HD textures to the game. I was able to search the forums and find literally dozens of texture packs to re-skin the game out of it's charmingly ugly pixel art and a quick check of the official wiki revealed countless other tools. It's also worth mentioning that while multiplayer is a brand new feature, there are already countless servers available which are independent and support different mods and community-run side projects.
You can try out the free version at the official site, but be aware, it doesn't do the full version any kind of real justice with only a fraction of the content. If you're the type that likes to create and explore you'll love this game. Also, the $14 you'll spend on it during it's alpha stage is well worth the numerous hours of digging, collecting, building, and yes, running from skeletons you'll get out of it.