Onigiri is really fun! It's a huge game, so you always have stuff to do, and it's free online and on the Xbox. It's an MMORPG with a single player mode (thank god), and there are so many quests. Some are really easy while others are really hard, so it works for a lot of different types of players.
I'm surprised it isn't more well known? Also, it's one of the less buggy MMORPGs, and it actually work pretty well on console. My only criticism is the limited options you have when designing your character. You can't even change your skin tone.. But other than that, the game is super fun, and I play for hours at a time ^^