Valve just released a massive update to their Source SDK, adding all the Team Fortress 2 client and server game code
This will allow content creators to build completely new games based on Team Fortress 2. Unlike the Steam Workshop or local content mods, this SDK gives mod makers the ability to change, extend or rewrite Team Fortress 2, making anything from small tweaks to complete conversions possible.
The SDK is licensed to users on a non-commercial basis, meaning that any mod created using the SDK must be free, and any content in those mods must be free. Team Fortress 2 mods may be published on the Steam Store, and after publication will appear as new games in the Steam game list.
The 2013 edition of the Source SDK is written in C++ and also contains the previously released game code for Half-Life 2 and Half-Life 2: Deathmatch.
Click the link below for more information about the Team Fortress 2 source code release:
https://www.teamfortress.com/post.php?id=238809
SDK Installation Guide:
https://developer.valvesoftware.com/wiki/Source_SDK_2013
Source SDK 2013 on GitHub:
https://github.com/ValveSoftware/source-sdk-2013
