I used to be a HUGE fan of UrbanRivals, the browser based MMOTCG, until I realised the embarrassing amount of money I was sinking into desperately collecting the cards. Then I had to go cold turkey and ensure that I never play a game with compulsive desire to collect an unachievable ammount of things again.
The game was good though... a surprising amount of depth for such a simple game. It started off as a simple guessing game of TopTrumps style play, but eventually became a game of deception and bluffing. It really was quite addictive.
I still have my account though... I wonder if there is a way of selling it. I have a shit tonne of rare, now unobtainable (outside of trade)limited edition and special edition cards that are probably worth quite a lot.
EDIT: I just went back onto my account. I haven't been on in years, and although 4 new clans have appeared in which I haven't got any of the cards for, I still have 80% of the cards in the game! :S I really did sink a lot of money into this!