I've had a few questions about and requests for iPad (etc.) apps related to Math - many looking for specific features or emphasis. I'm hoping to do more exploration in this area and provide more resources along these lines. In the mean time, I hope to share some links that might be helpful.
I found a browser based (internet) version of a "drag and drop" math tool. AND there is a version of it that is available as an app for iPad, iPhone, and iPod touch - for just $0.99!
online version:
http://www.mrnussbaum.com/draggable/index2.html
app purchase:
http://itunes.apple.com/us/app/dragndropmath/id428845625?mt=8&ls=1
Please keep tuning in for more reources!