This mod is plugin loader based on what Mojang said during the Plugin API panel at Minecon 2012. Everything is loaded when you load a world instead of when you open the game, and plugins never have to deal with number ids. In the future people will able to develop plugins for this mod. These plugins will be able to add, change, or remove features.
Just install it like any other vanilla mod. This doesn't require forge or any other mod loader.
In the create world menu, click the plugins button to enter the plugins menu. It works similarly to resource packs. To add a plugin, just click the arrow button to move it over selected plugins list.
The three plugins built in are: Green Blocks, No Explode Creepers, and Rotten Flesh Uses. Green Blocks adds three blocks: green wood, stone, and dirt. No Explode Creepers makes creepers no longer explode. Rotten Flesh Uses actually only adds one use for rotten flesh which is to cook it into cooked flesh. Cooked flesh doesn't give you the hunger effect, but it restores less hunger.
Questions you may have:
Q: Can I develop plugins? A: No, not yet. If I allowed plugins to be developed now, they would just break later.
Q: Does this work with multiplayer? A: Again, not yet. You can see that I disabled the multiplayer and realms buttons in the main menu.
Q: So, what can I do with this mod now? A: You can try out the 3 plugins built in to the mod right now. The point of this right now is to demonstrate how the official API may work if Mojang sticks with what they said at Minecon 2012.
Q: What will plugins for this mod be able to do in the future? A: Plugins will be able to add, change, or remove features. They will not be able to add menus or be able to change the base code in any way.
Q: What will be added to this mod in the near future? A: Multiplayer will be the next thing to be added, and after that, allowing people to develop plugins.
Eventually, you will be able to play minecraft without the minecraft plugin enabled.