This is Forge Mod Loader.

You can find the source code at all times at https://github.com/MinecraftForge/FML

This minecraft mod is a clean open source implementation of a mod loader for minecraft servers
and minecraft clients.

The code is authored by cpw.

It began by partially implementing an API defined by the client side ModLoader, authored by Risugami.
http://www.minecraftforum.net/topic/75440-
This support has been dropped as of Minecraft release 1.7, as Risugami no longer maintains ModLoader.

It also contains suggestions and hints and generous helpings of code from LexManos, author of MinecraftForge.
http://www.minecraftforge.net/

Additionally, it contains an implementation of topological sort based on that 
published at http://keithschwarz.com/interesting/code/?dir=topological-sort

It also contains code from the Maven project for performing versioned dependency
resolution. http://maven.apache.org/

It also contains a partial repackaging of the javaxdelta library from http://sourceforge.net/projects/javaxdelta/
with credit to it's authors.

Forge Mod Loader downloads components from the Minecraft Coder Pack
(http://mcp.ocean-labs.de/index.php/Main_Page) with kind permission from the MCP team.

