Are you a content creator for Minecraft, building mods for various creative ideas, to add rich new content to the gloriously varied Minecraft modding community? Then you might know about the Player API, which gives the modders that much needed push to get some of the mods to work in general. This gives you access to client, server, and bukkit classes and simultaneously reduces incompatibility between mods. Many very popular mods like Smart Moving and Aether Mod rely on the Player API to function which in turn makes this a lot like Forge or Modloader in that regard.
If you want to use this mod for creation, then there are many, many more details located at this link, examples of hooks and writing the code in order for Minecraft to function with your mod properly. Do you need your player to be able to have new animations or move differently? This is just one example of what you can do with such control over the classes.