"There are still (human) populated servers (at right times of the day)!" Edition
Originally released as freeware, later got open-sourced (GPLv3) like most id games of the time.
Team based, objective oriented, WW2 themed FPS.
Uses further developed idTech3 (quake3) engine so all the movement mechanics are there. Trickjumping and defrag exists.
If you run original game you might encounter some problems (especially sound on Linux) so it's recommended that you use a source port like etlegacy.
Etlegacy fixes a lot of problems and adds more options while maintaining complete 2.60 compatibility except PunkBuster[1] and ETPro(other mods to substitute?).
Download here[2]:
http://etlegacy.com/The way I installed it on Linux is I simply extracted downloaded etlegacy binaries over original 2.60 ET folder[3].
If you use Arch Linux, AUR version works too.
Readme and AUR page will give you an idea of dependencies to install.
64bit version is incompatible with 32bit. Since original game is only 32bit, use that, else, you will be way lonelier.
[1] PunkBuster ended support and doesn't do shit in terms of cheat prevention these days, some popular mods like Silent and NitMod incorporate better anti-cheat anyway, not to mention cheating is not a big problem really.
[2] If you go to wiki's download page there is a frigin' AROS(amiga port?) build there.
[3]
http://www.splashdamage.com/content/download-wolfenstein-enemy-territory