I personally enjoy a little bit of Happy Wars every so often. I find the game to be fun when played in small sessions. I also hear a lot of people enjoy World of Tanks. Gotham City Imposters is also free this month and is a decent Online First Person Shooter. Since there a other Free-to-Play games available you could just try out the ones you think are interesting.
I was really into Happy Wars for awhile. Clerics rule.
It does have the same problem as all MP games though, in that 90% of the time, your teammates are idiots who won't help at all with anything. --- Not changing this sig until the Detroit Lions win the Super Bowl. GT- ShinigamiOfTime
Like said before, having friends help the f2p games as your teammates you're randomly put together with will be no help. Warface is a pretty good game, WoT is decent but needs team communication, Happy Wars imo experience you need good items and teamwork to do anything, WSOP is a good game but I don't like poker that much.
F2P right now is GCI and it's my favorite of the group, but it's only until BBT is available, I've bought the game twice lol --- GT- Discern88