Valve releases TF2 SDK to the public for modders, allowing changes ranging from tweaks to complete conversions. Modders can create TF2 mods for non-commercial use only, must be free, and not ...
Valve has opened up Team Fortress 2's full client and source code, encouraging players to 'change, extend, or rewrite' the game in any way they like. However, all mods must be released for free.