You can now brew your very own (free) Valve-approved Steam games from the guts of Team Fortress 2

rockpapershotgun.com:

Valve have unleashed Team Fortress 2’s client and server game code into the wilds of the internet by way of a Source software development kit update, allowing modders to release their own Steam games based on the long-serving hat-powered FPS – as long as they give them away for free.

It's a step up in capacity from existing TF2 modding functionality. “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,” explains a blog post on the official site.

“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,” it continues. “TF2 mods may be published on the Steam Store, and after publication will appear as new games in the Steam game list.”

The post also notes that Team Fortress 2 players have spent a lot of money on stuff created by Steam Workshop contributors, and that “the majority of items in the game now are thanks to the hard work of the TF2 community". Valve would like Team Fortress 2 mod makers to “respect that connection” by not making mods “that have the purpose of trying to profit off Workshop contributors' efforts”. In other words, you're not allowed to re-monetise a decade of community-created hats. Valve would love for all mods to let players access their existing TF2 inventories freely, “if this makes sense”.

The release of the client and server code accompanies a big update to back catalogue Source engine titles, including Half-Life 2: Deathmatch, Counter-Strike: Source, Day Of Defeat: Source, and Half-Life Deathmatch: Source. This adds “64-bit binary support, a scalable HUD/UI, prediction fixes, and a lot of other improvements”.

Team Fortress 2 was originally released in 2007, back when there was still some hope, and remains one of the top 100 most-played games on Steam as of today. It’s also one of our favourite shooters