Maplestory. It's a MMORPG, and there are a ton of classes you can be. (Warrior, Magician , etc.) You can buy stuff with NX, which can be bought with real money, but you can earn it doing surveys too. It's the best free game ever in my opinion.
Free games. There are a ton, and as far as my favorite....I don't really have one. I play Starcraft2. That's about it, and when I don't play SC2 and want to play something I come to AG for a few quick games of whatever is new or sometimes on long dark and cold nights after a few drinks I search the deep archives of AG and play their oldest titles.