see LinuxGames too or LinuxTests

Unreal Tournament

only demo available gratis
http://www.beyondunreal.com/main/ut2004/ut2004essential.php [en] download essential for ut2004 under GNU/Linux
http://www.linuxelectrons.com/article.php/20040419200456201 [en] installation and benchmark of unreal tournament 2004 under GNU/Linux (with ATI graphic card though, nVidia is recommended in OpenGL

http://wiki.beyondunreal.com/wiki/ [en] unreal wiki

http://www.pcgamemods.com/ [en] mods for UT (requires commercial version, I suppose)

http://www.linux-gamers.net/modules/wfsection/article.php?articleid=40 UT2004 Linux Tweaking Guide

Ennemy Territory

available gratis to play online

others

http://www.battlefront.com/products/dropteam/news.html check license

https://linuxfr.org/~gart/19585.html [fr] Quake 4 et Serious Sam 2 available for GNU/Linux

https://linuxfr.org/~farvardin/20156.html [fr] proprietary games

http://linuxfr.org/~phhusson/21255.html [fr] both free and non-free, Torus Troopers (BSD) compiled with D (non-free)

Tutorials

http://linuxaide.free.fr/articles.php?lng=fr&pg=49 [fr] cedega...

"Games" with bogus drivers

or just blame proprietary drivers, their blob firmware and their unwillingness to understand or contribute coherently to free software, shame is on them
http://ati.cchtml.com/buglist.cgi?query_format=specific&order=relevance+desc&bug_status=__open__&product=&content= [en] bugzilla for ati drivers for Linux
http://wiki.cchtml.com/index.php/Frequently_Asked_Questions [en] why ati driver is buggy

http://nouveau.freedesktop.org/wiki/RequiredFunctionality [en] if only nvidia *really* contributed /o\

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