I'm amazed by just the amount of applications available for this one tool. I was at the AMDM conference last week and half of the room pulled out their iPhone to share this "really cool app". Of course, if you aren't a math teacher I very much doubt that you would think it is cool. One of the teachers then hooked their iPhone up to the projector and showed the rest of the class the "math ref" app. Anything you wanted to know about math could be found at this on app. The teacher commented that the students in his class ask him if they can check to see if they are doing their work correctly by opening up "math ref" on their own phones. I love the way that he described his students using the tools in the their own back pocket to help them succeed in class.
I did like the fact that there was also a graphing calculator app for the students that do not have their own graphing calcualtors.
I really enjoyed looking through all of the apps available for the i-products. I didn't find many free ones for math that I really liked, though. However, the most expensive app I saw was $1.99.