Free Realms is a free MMO offered by sony (it has pay aspects, but is not needed) it takes an interesting approach to MMOs from what i've seen. It also basically plays from your browser. There is a small download, but it puts an app into your browser to launch the game. it also only has the main loading screen and then loads as the game runs. It is designed as kid friendly, but that doesn't take away from the fun (just no undead ugly scary monsters).
Now the real kicker for me and the reason i'm posting is that it has a TCG within the MMO (sony likes to do this, there is a TCG w/in Everquest and SW:Galaxies). You can play in the TCG absolutly free, and get cards (solid ones) by following the Card Duelest career path (career paths are open, no need to select one at the begining) and the quests associated with it. It comes with the nature starter (or at least mine did) which is a surprisingly tight and focused deck for a starter. I've only had trouble with one deck in the quest so far and that had more to do with 2 bad plays by me.
You can buy packs for the game if you want (its a two step processes as you have to buy the game's currency and then use that to buy the packs) or you can become a member (epseically if you like the MMO as a whole) and get a free pack every month as a member bonus.
The TCG itself plays a little llike Cthulu in that you use cards in your hand to fuel your resources (place them face up or face down in the resource area, face up cards can provide a bonus from the resource area if their game text says so, otherwise placed facedown) so it ramps up the same way as CoC. The strategy is not deep, but its fun and there are a lot of combos. I'm enjoying it.