Khan Academy features some incredible videos by experts hoping to educate viewers about anything and everything they might find interesting.

http://itunes.apple.com/us/app/khan-academy/id469863705?mt=8

While mainly targeting kids, critics adore MathBoard for its accessible lessons in addition, subtraction, multiplication, and division. Lifelong learners needing a little push in the math department might want to check out what it has to offer as a refresher.

iTunes: http://itunes.apple.com/us/app/mathboard/id373909837?mt=8