Depends what you are looking for, there are some good ones which have games in them. I have bought iAdvent which has some word games and Bloons festive fun, which gives bloons levels each day. I'd recommend these two, Bloons is more fun though
Here's a great one that picks a favorite free game or app every day. Advent Calendar 2009 - Christmas Selection of the Best 25 Free Apps. They picked our game Harbor Master Free for the first day, which was really nice of them.
i know this is not an app but i think you'll thank me if you didn't know about this already http://appventcalendar.com/ you can get snorkeling, but you missed tridefense and 33rd divinsion. more going free for a day soon (i.e. every day until christmas eve)