They are all team based shooters that are played by rounds. Each has heavy load out customization with items that can be purchased via in game earned points or quicker via real money. I have put 10-15 dollars into each and found them to be good investments.
I play Pirates, Vikings, and Knights 2 http://www.pvkii.com/ ...an update (including a new class and maps) is supposedly very close (but you never know how long it'll take), with another one to follow a little bit later that will add another class that wasn't quite ready in time for the main patch.
My wife plays Lord of the Rings Online on occasions and she says its good.
I play Rift which is like $12 a month. Meh. Great game. Good value.
Age of Conan is the worst free to play game. Actually its the worst game of any kind. Period.