iPhones and infants

I've realized that there is a growing demand for information for apps for what I'm calling iPhone Babies. I'm in the process of testing other apps with my baby girl (but Peekaboo Barn is still the take all home run).  What I'm looking for are suggestions for apps.  What does your baby enjoy, what gets their attention?  Paid or free, doesn't matter, I'm curious to see what other magic apps are out there for us tech savvy parents.  Feel free to comment below on what apps you feel fall under the category of "Must Have".