I like free stuff and I want to know what is good.
Well, they are not entirely free, because they are mods for games.
But DayZ is a good mod for ARMA2: Combined operations. You can sometimes pick that up quite cheap on Steam.
The other mod I would recommend is for Medieval 2 TW Kingdoms. It's called Third Age Total War, and it's basically a LOTR mod, if you haven't already guessed. Again, because it is quite an old game, it can be bought quite cheap on Steam.
hidden source, red eclipse, warframe, planetside 2
overlord has already mentioned redeclipse...
and hundreds of other free and open source games!
Look for a list on the repos of the polish distro Angel Linux, it's a gaming distro and they have a large collection of open source games ready to run out of the box. Other gaming distros to get lists of open source games from include: Arch/Manjaro: easily the largest collection of up to date open source games period, especially with the AUR taken into account, Fedora games spin (based on Fedora 17, so not supported anymore, but still available and the games list is on the spin website page), etc...
War thunder is a good f2p.
Hawken, Its good but pretty bad optimisation for Amd cards.
Falskaar, a mod for skyrim that add's alot more gameplay to the already long game.
Gmod/CsSource mods: Hidden, Prop hunt, Trouble in terrorist town.
Full invasion mod, a mod for mount and blade that makes it a co-op survival.
Planetside 2, good game if your pc can run it well.
World of Tanks is a free game I've played more than any other game I own.