Disclaimer: This is my preferred shortlist to introduce the power of free to play. I did not mention a lot of popular free to play games here as there are nearly one million f2p games in pc platform.
These games are generally multiplayer and co-op only , you're gonna need a decent internet connection to play in foreign servers. Few games may let you to host your own server.
Battlefield 3 was free for a week last month(june), those who added the game in origin can play the multiplayer forever (I hope all of you added it).
Now I share some free to play original multiplayer games-
✔ Planetside 2 (well balanced sci-fi mmo fps)
✔ Ghost recon phantoms (great tactical thirdperson shooter but ping problem as all servers are in eu)
✔ Backlight Retribution
✔ Warthunder (if u like dogfighting with jets)
✔ Hawken (mech battles)
✔ Heroes and Generals (Like cod2 with vehicles, new world war fps+strategy)
✔ Unturned (survival game, graphics like minecraft
✔ Warface (new fps with good graphics, shooting like cod/bf))
✔ Tribes ascend (movement too fast, may remind you of UT)
✔ Marvel Heroes
✔ Star Conflict
✔ Tactical intervention
✔ America's Army Proving grounds
✔ DCS World
last but The BEST- ✔✔ TEAM FORTRESS 2 (comment not needed, the game speaks for itself)
✔ Fifa world (those who wanna play fifa online but didnt buy fifa)
✔ NFS world (probably best f2p racer)
✔ World of warplanes
✔ World of tanks (if u like armor)
mmo i didnt mention- dota2,lol,SWToR, LotRO,DC universe, PoE and MANY MORE... just google for top f2p games.
also, f2p survival mmo H1Z1 is coming soon, I will be buying early release.