Not on steam and neither on gog I've 3 suggestions for multiplayer fps fans:
Xonotic
An open source arena FPS based upon a modernized Quake 1 engine, it can be described with "quake feeling with unreal tournament weapon combos"
Renegade X
A fan made game, authorised by EA till it's free, with the aim to recreate and improve from ground up the c&c: Renegade shooter that failed a long time ago.
Project Reality
A standalone Battlefield 2 Total Conversion that does miracles with its 12 years old engine. PR is a large scale 50v50 tactical fps with a really strict set of rules (manual reading is necessary), with maps inspired by some real life warfare, military roleplaying at its best!
How is this post any relevant to this thread? This thread is about game giveaways and generally freebies for things that otherwise are paid, not for freemium games.