Guardian Cross
An app for iphone / ipod by SQUARE ENIX
Guardian Cross is a creature-based card game that features a fully unique battle system in which players take on the role of “tamers” who capture, raise, and control powerful beasts known as “Guardians.”In addition to quests to unlock the secret of why the Guardians have woken after a millennium of slumber, other features include a coliseum system that allows players to challenge other tamers from around the world in head-to-head combat.
What makes this game interesting is that not only is it a card based game, but in order to get more cards, you have to play a mini game where you have to hunt creatures to turn them into cards.
They update this game really often and every week there is a pvp tournament sorta thing to see how good you are. The higher rank you place, the better the rewards. Interesting to see how well you do amongst everyone else. From what I observed, they've been adding a lot of special weekly events and they keep updating the quests available as well.
The game is FREE, so give it a try!