[Minecraft] is very moddable. While it doesn't have official mod support (yet) it does have a huge modding community with tons of information and user-created tools that can be used to mod Minecraft. Some of the most impressive mods have even been added to the official minecraft release, such as the first Smooth Lighting mod by whatshisname. :nod:
I know - I have the http://www.minecraftwiki.net/wiki/Minecraft_Coder_Pack and use it to play a bit with the game. It's just a bit of a pain to set up initially, and also not really easy to use the mods, at least until ModLoader becomes a standard part of the game.