Wizard 101 is a game primarily for younger players and even children but somehow it is addictive for adults as well. While playing it I've met lots of other adults, though. :)
The game is (at least the German version I'm playing) fully synchronized which I think is very rare for a F2P game. Furthermore the game is a little bit like a an addicting trading card, because the magic you're using is based on cards. Combat is max. 4v4 and you fight through "summoning" beasts through the cards. I think there is no other game like this right now which makes if kind on unique.
Sounds interesting? I created some videos showing some of the boss fights in this game:

http://www.digioso.org/Wizard101 - Krokopatra
http://www.digioso.org/Wizard101 - Pops O leary
http://www.digioso.org/Wizard101 - Miauriarty