Costume Quest has been out for a couple of days now on Steam, but it's never too late to pick up the game and play it. Having enjoyed it on the PlayStation 3 when it was released last year, I can say it's easily one of the best-written, and funniest games I've ever played.
In Costume Quest, you'll make your way through the game as either a girl or a boy to rescue your missing sibling. Along the way, you'll pick up allies and build your assortment of costumes, which you can deck out with flair for added bonuses. Each costume you possess allows you to traverse through the game's various challenges.
The combat—which is a bit like Final Fantasy with quicktime events for limit breaks—isn't too special, but it doesn't get in the way of the game.
Costume Quest on Steam also includes the Grubbins on Ice DLC which is still sold separately on the console versions of the game. Go play it!