There are plenty of apps that teach kids their letters, but Intro to Letters, by Montessorium ($5) instructs tots to recognize, pronounce, and write letters, as well as simple letter combinations (aka phonograms). When kids trace the letters, the app says it's sound. It also says the letter's name and allows kids to record themselves saying the sound to see how close they get to the original. One of the most popular apps in the App Store, Intro to Letters was featured in an Apple ad that aired on national TV.
Intro to Letters, by Montessorium