iOS

http://speirs.org/

http://maineeceipad.wikispaces.com/

For the higher end of K-12 I would look at SpaceTime.
http://itunes.apple.com/us/app/spacetime-scientific-computing/id293619493?mt=8

An app I like and my children like is Mathemagics - <http://itunes.apple.com/us/app/mathemagics-mental-math-tricks/id306586847?mt=8]]>

Wolfram Algebra:
http://itunes.apple.com/us/app/wolfram-algebra-course-assistant/id410592976?mt=8

Wolfram Calculus:
http://itunes.apple.com/us/app/wolfram-calculus-course-assistant/id410594105?mt=8

Both allow a student to input a problem and see step by step how the answer (for example taking a derivative or expanding the product of two expressions) was derived.

http://a4cwsn.com/
http://www.autismspeaks.org/community/resources/apps.php
http://itaalk.org/home.html

http://web.me.com/dowd/Touch/Welcome.html