Forge

Official Forge download page (all releases)
Minecraft Forge is a modding API for clients and servers.

Instructions

No special instructions for Forge! You should read the instructions (Forge and Minecraft 1.6) at their website, but there's a summary
  • Download the Forge installer (main download)
  • Run it. This creates a "profile" in the Minecraft Launcher
  • Restart/Refresh the Minecraft Launcher. You should see a "Forge" profile
  • Start it!