What you’re allowed to do with minecraft
If you’ve bought the game, you can do pretty much anything, but there’s one major rule:
Do not distribute anything I’ve made, or modified versions of anything I’ve made. I want all client and server
downloads to come from minecraft.net.
I don’t wish to lose control of this project, and definitely not before
it’s been finished.
As pointed out on irc, this clashes a bit with modding support at the moment, but some kind of server api is planned and coming sooner or later when I feel better.
So keep in mind that distributing [modified] clients and servers not only is illegal, it’s against my wishes.
This said, if you write a custom server and let people play on it, that’s very cool and just adds content to the game. =D