I found out about Minecraft sometime late last year and I'd highly recommend it to anyone who enjoys these types of games. Right now Notch (the sole developer)

I found out about Minecraft sometime late last year and I'd highly recommend it to anyone who enjoys these types of games. Right now Notch (the sole developer) is working on adding multi-player support to the latest version of the "survival" game. Essentially, there are two versions of the game, one is a sandbox mode where you can build anything you want with unlimited resources and the other is a survival mode where you start out with nothing and have to collect wood to create tools to collect stone and so on. Even without any external interaction right now it's amazingly fun to create huge fortresses in the mountains and cliffs that are generated with a new level. He's also very open with the community and readily takes suggestions and implements them. At one point he'd released the map format (based on his custom NBT format) so community developers could create tools of their own. I'm sure the format has changed drastically by now, though, since he changed the levels from a fixed size (64x64x32 blocks or variations of that) to an unlimited world size. I can imagine multi-player servers for this springing up and having huge cities being built by players.