Saw this site called AppsAmuck which provides 31 example applications. Just started yesterday and up to the 2nd one now. It's quite interesting as the site provides detail information plus the source code - although it's a bit out of date.
The 1st example is about showing a clock counting down to midnight. As shown below, the code can't compile in XCode 4, after some research on the net I got it working and added another clock on top - one showing the current time, the other showing the count down. I think this would be quite handy if you need to write some game and have a count down displayed somewhere.
The 2nd one is about animation. As the old code still compiles, I was a bit lazy and just played around with it, added another animation I downloaded somewhere (I forgot :-P ) about street fighter. Quite interesting watching it punching left, right and center above the lively fire background.
Wonder how many days can I go on, I think everyone should have a try!