So for our first app of the site we have Preschool Adventure from 3Dal LLC. 
This app is an entertaining choice, and some actual learning too. It teaches some hand eye coordination, colors, shapes, animals, and animal sounds. How long does my 2 1/2 year old like to play with it? About 10 minutes or more. She is adept at pressing the X to close the current activity as well as choosing another. The learning is colors and shapes although the style does not lend itself to great retention by the child.
Here is how I rank it:
Learning: 4/5
Fun: 4/5
Attention Capture: 5/5
Not too bad at 13/15.
Age: 5 and under
You can pick it up in the app store here: http://www.itunes.com/apps/preschooladventure
You can visit the creator’s website here: http://www.3dal.com/
Some images from the app:
A video of the Preschool Adventure iPhone app being used by a 2 1/2 year old: