Free Game - Puzzle Pirates
One of the more popular free games on the 'net, Puzzle Pirates is a persistent MMO with a very casual bent. You create a cute pirate that looks like a Playmobile figure, and sail the seven seas in search of pieces 'o eight. The catch is, each activity (and there are many, from manning the guns to repairing the ship to bilging out water) is performed with a simple puzzle game. Everyone on the ship plays their puzzle at the same time, and the cumulative performance determines how well your ship does. There are tons of community features, lots of help and tutorials, tournaments, clans, etc. There is a money angle, though—you can play for free forever, but you can also purchase a subscription or buy doubloons to access more advanced features.
You may also download the game to your desktop.