Natural Selection 2 [Steam] is the FPS/RTS hybrid released for Linux way back in 2013, many years later it's still getting updates!
This latest release adds in a new tutorial named "Skulk Challenge", which will help you learn the various abilities it has like wall jumping. It also serves as a challenge mode, since you can download ghosts of other players and compete against them and earn badges and get a global rank.
They finally updated the confusing spectator system too. Spectators now have dedicated slots, meaning server owners can have full 24 player servers with 24 spectator slots too. A much nicer system, as I was often confused when trying to join a game and seeing "25/24" or something.
It took a long time, but 64bit is officially going into beta and will be released as a small update. Their 32bit version will officially retire, so if you're playing and you're on 32bit, you should seriously upgrade already.
On top of that, they also polished the game as a whole with balance fixes, bug fixes and so on. You can read the full changelog post here.
Anyone here still playing it? There's still a few hundred people playing it every day. It's also rather cheap now, at £6.99 it's not a bad price.