Back before pirates got cool thanks to a certain Mr Depp, Three Ring's Puzzle Pirates launched itself onto the MMO landscape. You control a pirate avatar who can perform tasks on a ship (and elsewhere) through participating in a certain types of mini-games and thus contribute to the overall benefit of the crew you are sailing with. Carpentry is a 'use certain fixed shapes to fill in holes', sailing is 'get the right coloured circles into the right coloured markers', bilging is 'line up the right coloured icons to score', and so on. There are a large number of mini-games within Puzzle Pirates, each different enough that I didn't see the same theme repeated.
The fun-nest part of Puzzle Pirates I could experience (since the Adventures component always seemed to hang to infinity) was going on a mission with a pirate crew - you'd sail around performing tasks on board in order to help the ship move efficiently, then run into a combat situation where one ship fights another ship. Sadly, I sucked very badly at the Tetris-esque sword fighting mini-game, but was on a few ships that won and got a share of sweet pirate loot.
The biggest barrier to me getting deeper into Puzzle Pirates - and the time seemed to fly by pretty quickly when I played, let me tell you - is that the part I enjoyed most was the part I also sucked at, i.e. the sword fighting and taking other ships. But that doesn't mean I'm going to abandon the game. Puzzle Pirates was enjoyable enough for me to put it on my back-up MMO list - whenever I need a break from my main MMO, or don't have a game to play, Puzzle Pirates seems like the perfect light entertainment to spend my time in. At some point I'd have to start paying for it, I know, but until that point, I'm happy to play on one of the Doubloon oceans for free... or for the occasional micro-payment.
If you want something light and fun, give Puzzle Pirates a shot.