Mojang is currently developing the Minecraft Plug-in API, which is a common set of tools that will allow plug-in/mod-makers to interact with the game. There are also plans to create a central repository of plug-ins. For information about how this will be implemented, you can watch the plug-in talk from Minecon 2012.
Modding is not officially supported, nor can we provide support for mods. Modding can be complex, and may cause your game to no longer be playable. However, if you would like to try Minecraft mods through the new Minecraft game launcher, the following two videos should help explain how to do so.
For help with mods, you may want to visit the Minecraft Forums for topics and discussion with other Minecraft