Here are all the apps we use on our ipods.  Some of the apps are more appropriate for other grade levels, but for the most part, 2nd graders can use almost all of them.  The ones in bold are the ones that I like the best for our current second grade math concepts.

Math

Candy Factory Education
Cloud mathCounting Bills & Coins
Estimate Fractions
Fraction Drills
Math Train Free
Math Tutor
Pizza Fractions
Pop Math Lite
Number LineMath Drills
My Math App
Units
Rocket Math Free
Shapes Telling time

Literacy

ABC Magic Phonics
ABC Magic 2
Aesops Quest
Alphabet Tracing
Articulate!
Connect the Dots lite - Sea Life
Dragon Dictation
Grammar Dragon
Magic Sentences
Opposite Ocean
Same Meaning (1 and 2)
Same Sound Spellbound
Time Reading lite Vocab Daily
Vocabulary Bubble Free
Word magic lite