http://www.minecraft.net/ It's something like a cross between Legos, Harvest Moon, and a Roguelike RPG.
It's good, and it's only in alpha right now.
It has freeform building for newbs and arteests.
It has multiplayer (in creative mode).
It has a single player survival mode that's both interesting and requires some thinking. (the later finished version will have lots more to it.)
Java required & currently browser-based, free-to-play modes available, though if you drop $14, they let you play the in-development version which has more depth. Even so, it is simple and also nifty.
Sometimes it takes a while to load, though, so open another window and browse while you wait.
It's only in alpha, y'know?