Valve: > Unlike the Steam Workshop or local content mods, this SDK gives mod makers the ability to change, extend or rewrite TF2, making anything from small tweaks to complete conversions possible. > The SDK is licensed to users on a non-commercial basis,

Valve have unleashed Team Fortress 2’s client and server game code into the wilds of the internet by way of a Source

Valve officially dropped the software development kit for the long-beloved multiplayer shooter game for free in its latest Source SDK update.