First off, I love all of the ABA Flashcards, ABA Receptive Identification, and ABA Problem Solving Games by Kindergarten.com.
My Spelling Test is another favorite of mine since all of my kids who are able to spell are at different levels. I record myself saying the spelling words of each student's list. Later, when they take their test, they hear my recorded voice saying their spelling word, and then they have type it into a text box.
iMovie. We have used this for fun to make movie trailers. There are many different templates you can use like fairytale, pirate movie, scary movie, etc.
Numbers. I can't even begin to tell you how much I love this app. I use this to record all my data on each student. I create a page for each kid, and within their page I have a tab for each IEP goal. It has tables and graphs. This is great for sharing with parents and making data based decisions as you go, instead of waiting a month until you have time to graph your data.