Directly form Facepunch's thread:
Have you ever felt uncertain about the direction TF2 is heading in? Or perhaps been disappointed that Valve hasn't given this game the attention it deserves? Never fret, your worries have been assuaged - our team has begun work on a community-driven edition of Team Fortress 2!
No, you didn't misinterpret that - we have "acquired" the full source code of the game to modify it as we please. Withhold your enthusiasm for another brief moment - our code is from 2008, before payload even existed. But with our development team and your gameplay, feedback, and enthusiasm, we will be well on our way to producing a version of TF2 that this community can come to love and enjoy!
Installation Instructions:
1. Download the client and the latest patch
2. Once downloaded, extract everything to your "steamapps/sourcemods" directory. You should now have "steamapps/sourcemods/tf2classic"
3. Go to the 'Tools' section in your Steam client
4. Install Source SDK 2013 Base Multiplayer
5. After it's installed, right click it and select Properties, then click the Betas tab and choose "Upcoming".
6. Close the properties and it should begin updating.
7. Once it has finished updating, restart Steam
8. Check your games. You should see "Team Fortress 2 Classic" has appeared on your games library.
Autoupdater program now available, click here!
Steam group
GitHub
Bug report