I've tried a few and some are great while others are just clones of exisiting games. The thing is because they are free there is an expectation that they're shit 
and in some cases this is true, but not always.
If something is free the value of the game is already compromised. In the west, we pay for our games, well at least the MP ones where there is a chance at a community and some eSports action. However, in asia and other nations outside of western markets, they tend to promote free games, which are mostly free on the surface, but within the game itself they offer micro transactions to promote a deepr gameplay experience.
In the west, some gaming companies are trying to adopt the free concept and micro transactions. In some rare cases we have seen companies give away there MP portion of there game out of good will. I've done a quick run around to find some free FPS games out there. If you're feeling frugal this Xmas, download a free of these, whack them on a disc and give them to loved ones. 
Okay first comes the CS clones, argueably Combat Arms is the best of these
Combat Arms
Crossfire
Blackshot - - blackshot.outspark.com/
AVA Alliance of Valiant Arms - Urban Operation
Soldier Front
America's Army 3
FEAR - projectorigin.warnerbros.com/fearcombat/main
Wolfenstein Enemy Territory
Battlefield:Heroes
Quake LIVE
Hidden & Dangerous
Kuma War
2War
Postal
Savage 2: A Tortured Soul
Urban Terror
Tribes 2
WarRock
Warsow
Gunz: The Duel
KARMA: Operation Barbarossa
Alien Arena
D-Day Normandy
True Combat: Elite
Huxley:The Dystopia
Coming soon:
Parabellum - unreal 3 engine. parabellum.xg1.net/
Great articles on Free FPS games:
www.marcofolio.net/games/the_best_free_g...on_shooters_fps.html
www.1up.com/do/feature?pager.offset=4&cId=3174065
freefps.blogspot.com/