Minecraft never got an open source release or official mod support. Mods are made using libraries reverse engineered from the official game, then installed by patching the game. (This is made a bit easier than it would otherwise be because Minecraft is written in Java, but Mojang runs it thru a code obfuscator so unzipping the .jar shows a huge codebase with class and method names like "Ijf8". It's pretty gross.)
MineTest is open source and has modding support as a feature.
MineTest is open source and has modding support as a feature.