Team Fortress 2 Classic is the re-imagining of the 2008-2009 era of the original Team Fortress 2 designed as a casual mod that anyone can get into and enjoy a little FPS action.
It released properly back in July to much excitement in the TF2 modding community and it sounds like it's doing well. The release came with a few Linux issues and problems elsewhere, which have been fixed up with a new patch out now. On the Linux side, it should fix broken sounds and a crash involving the Capture The Flag game mode.
For the Linux version, you will likely need to add this as a launch option:
Yes, twice. Otherwise it appears to launch in untrusted mode and you can't join VAC servers.
It's a fun mod and well made. Especially interesting timing now that Valve rarely do anything with TF2, so this could help bring back some of its former glory perhaps. To play it, you need to make sure you have the "Source SDK Base 2013 Multiplayer" installed and ready in your Steam Library. Then it needs some manual downloading of the full mod pack, to place into your "sourcemods" folder in your Steam Library. Hopefully they can eventually streamline the process a bit.