Valve just released the source code for Team Fortress 2

Valve releases Team Fortress 2 source code, opening door to completely new fan-made games
Team Fortress 2 maker Valve has handed over the game's full client and server code to fans, encouraging them to "change, extend or rewrite" TF2 in any non-commercial way possible.
Team Fortress 2 Modders Rejoice as Valve Releases Full Client and Server Game Code
Valve said the update lets players build entirely new games from its source code, and unlike Steam Workshop or local content mods, it gives modders access to change, extend, and e
Team Fortress 2 SDK Released By Valve
Valve has released the Team Fortress 2 SDK, which means we can expect a load of new content from the modding community.
Valve Drops Full Team Fortress 2 SDK, Allowing Any Modder or Dev To Make (Free) Derivative Work
Valve officially dropped the software development kit for the long-beloved multiplayer shooter game for free in its latest Source SDK update.
Valve releases full Team Fortress 2 source code, allows modders to run wild
Games giant Valve might be winding down development on Team Fortress 2 for good. The developer of Portal 2, Half-Life: Alyx, and occasionally, Dota 2, has released a huge update to the Source Software Development Kit.
Valve’s Team Fortress 2 SDK Update Opens Doors for Game Mods
Valve has rolled out a significant update to its source SDK, granting the gaming community unprecedented access to Team Fortress 2 server code.
Valve says you’re now free to build your own games atop Team Fortress 2.
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 adds "all the Team Fortress 2 client and server game code" to its Source mod tools, letting modders "build completely new games based on TF2" and publish the…
"We're already preparing for the porting efforts and a potential Steam release now that we've been legally enabled to pursue ...
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Valve releases 'Team Fortress 2 SDK,' enabling creators to 'build completely new games based on TF2'
Valve said it's "also doing a big update to all our multiplayer back-catalogue Source engine titles (TF2, DoD:S, HL2:DM, CS:S, and HLDM:S), adding 64-bit binary support, a scalable HUD/UI ...