me and a buddy of mine have been playing a lot of global agenda recently on lan.
it's a free mmo where you pick one of 4 classes - healer, tank, assassin, gadget, and play either co-op pve or pvp.
each class has it's own skill tree that you work up as you level, as well as their own gear and abilities.
basically, you can roam around outside the hub city and kill stuff, doing little pve quest missions, to level, or at any time you can launch into 4 player squad mini-dungeons on a timer, among other things.
the squad side of it can be pugs, or you can form your own team -- they have the usual agency (guild) side of it, too, with agencies battling it out in another aspect of the game.
it's not a particularly complex game, and got a metacritic score of 70, if that means anything, but I find I can play the crap out of it over and over --- if you get tired of one character you can always try working up a different class, and I find the squad pugs to be endlessly replayable.
haven't even joined any agencies, and while the squad pvp is pretty fun, I usually get destroyed.
here's an outdated vid review from a couple retards.:
http://www.giantbomb.com/quick-look-global-agenda/17-1964/
check out the free version if you've got nothin' going on --- just don't expect goty.