We just got Endless ABC's. The letters make sounds as you move them to their spots in vocabulary words that are pretty unique. They also explain what the word means once you match all the letters. Examples of words: Yodel, Artist, Belch, Question....
Re: Apps
I can't speak to which would be appropriate for visual impairment, but some that DS loves/loved:
Monkey Preschool Lunchbox
Grandpa's Workshop
Good Night Moon
The Monster at the End of the Book
Curious George Drawing