Free-to-Play Game Recommendations

We have a group of gamers at work that want to put together a list of good Free-to-Play or extremely cheap multiplayer games that we could choose from and play once a week. Maybe a new game every month or few weeks switch it up.

With Steam sales we don't mind finding a $5 or less game if everyone is on board, but prefer Free-to-Play.

Anyone have any suggestions? PlanetSide 2 is already on the list and we've played Team Fortress 2 quite a bit already.
 
Solution
I've been having fun with Heroes & Generals, and War Thunder. League of Legends is huge. Those first two games appear to have a bit of a glass ceiling in that you are able to obtain everything a payed account gets but it takes a longtime after playing for a couple weeks, to further your character without monetary help but I am still playing both games for a few months now without spending any money.

League of Legends is a bit easier on the player in this regard. Building up a Rune collection takes a little while but the game can still be enjoyed without feeling you are at a major disadvantage, especially in ARAM style play(All Random- All Mid).
I've been having fun with Heroes & Generals, and War Thunder. League of Legends is huge. Those first two games appear to have a bit of a glass ceiling in that you are able to obtain everything a payed account gets but it takes a longtime after playing for a couple weeks, to further your character without monetary help but I am still playing both games for a few months now without spending any money.

League of Legends is a bit easier on the player in this regard. Building up a Rune collection takes a little while but the game can still be enjoyed without feeling you are at a major disadvantage, especially in ARAM style play(All Random- All Mid).
 
Solution