It looks like the modding API for minecraft will be handled by some experienced hands. Bukkit member EvilSeph annouced that the Bukkit team has officially joined the guys at Mojang. Bukkit has provided by far the most popular minecraft server mod since late 2010, with hundreds of plugins and over a year working on minecraft from the outside I'm excited to see where they can take it with direct access to the internal workings of minecraft and the backing from the guys at Mojang.
You can read the post from evilseph here and the annoucement from Mojang here.
When minecraft first came out the player had to stop in the middle of the game in order to download various mods that make the game more interesting. With the ModLoader tool it has become much easier to create and use multiple mods in minecraft without worrying about conflicts between mods. The ModLoader enables the player the ability to use mods that are compatible with each other, thus giving them an improved gaming experience. Since the ModLoader has not yet been completely perfected, there continues to be some conflicts when using the same block ID's. Since this is the case it's best to use the latest version of ModLoader. The mods each player chooses to use will tell which version is needed.
1) Go to start>run, then type in %appdata%
2) Open the folder .minecraft then the folder bin
3) Open up minecraft.jar with either WinRAR or 7zip.
4) Drag and drop the files from Modloader.zip into the minecraft.jar file.
5) Delete the META-INF folder inside the minecraft.jar file.
6) Close the file and run Minecraft
1) Go to Applications -> Utilities and select terminal.
2) Type in the following into the terminal:
cd ~
mkdir mctmp
cd mctmp
jar xf ~/Library/Application\ Support/minecraft/bin/minecraft.jar
3) Outside of terminal, copy all the files and folders into the mctmp directory.
4) Back inside terminal, type in the following:
rm META-INF/MOJANG_C.*
jar uf ~/Library/Application\ Support/minecraft/bin/minecraft.jar ./
cd ..
rm -rf mctmp
5) Run Minecraft, enjoy!