Yohoho! Puzzle Pirates is one of those rare MMORPGs that manages to break the mold--and I mean it actually breaks it, instead of just taking bits from other games and sticking them together. In Y!PP, you play an adorable cartoony pirate in an adorable cartoony world who wins his fame and fortune though his prowess at various puzzle games. Y!PP is more open-ended than many games out there: you're not forced to collect four Giant Rat Tails and bring them to the captain of the Fellonian guard in exchange for a Very Smelly Cheese, which you need to give to the Elf Queen of Sapwood so she can make you some Pink Boots of Nancing...and so on. No, the game knows that you're smart enough to forge your own destiny, and there are many ways in which you can do that. Of course, many people aspire to captainship, but you can also become a trader, a shop owner (the economy is completely player-run), a titled noble in a Flag (a confederacy of pirate crews), or even the Governor of an island. If you're not into all that work, you can hang out at the inn and earn your Pieces of Eight through gambling on poker and swordfights.
One thing that's nice is that Y!PP players are almost never assholes. Sure, some new players ("greenies," they're called) can be stupid and mean, but they usually run off when they realize that their whining and posturing won't get them anywhere. Players rarely use chatspeak when they talk to each other, and you can often sustain a semi-intelligent conversation. Plus, Y!PP is one of those rare games where there is a near 1:1 ratio of women to men, meaning that the "boy's club" mentality found in most games does not prevail.
There are 5 servers, or "oceans," that you can play on. I recommend one of the subscription oceans (Midnight and Cobalt) for the rich and lazy, and one of the doubloon oceans (Viridian, Sage, and Hunter) for the poor and hardworking. If you want to carve out a big slice of the political and/or economic pie, I recommend the Hunter ocean: it only went live about two weeks ago and there's still plenty of ground to claim. You can find me on the Hunter ocean, under the name of Folie, so send me a /tell if you have any questions. Last week, I introduced Emo to Y!PP and he took to it right away. He also plays on the Hunter ocean, under the name of Emosamurai.
EDIT: Did I mention that it can be played on Mac and Linux?