It may seem a little unnecessary for a game all about building stuff to need its own mod tools, but mod tools are exactly what indie smash hit Minecraft is getting.
Getting hold of the necessary tools will cost you nothing, but you sign up knowing that the actual Minecraft has the right to use your ideas in the full game should it turn out to be a suitably awesome idea.
Any mods created will only be playable by those who have the full, paid version of Minecraft, and they can't be sold unless a licensing deal is worked out between the mod's creators and Minecraft developers Mojang.
You can read all about the nitty gritty at the link below.
The plan for mods [Notch]