Working in education has made me bias towards good educational apps. 'Endless Alphabet' is one of the great apps that combines great fun for kids with educational things such as spelling and vocabulary development.
(Ok I admit, I have played this app when my daughter wasn't around.)
Drag and drop the various pieces into the potato head then watch your creation interact on various backgrounds.
This app is endless fun for kids and a great free app to keep on your phone for those moments when you are in the grocery store check out line and you need your kid to kept him/herself amused for just a few minutes.