Most of us have memories, both fond and frustrating, of using graphing calculators in school. JsTIfied is a great webapp that can emulate the most popular models.
To use JsTIfied, you'll need to supply it with a ROM image for the calculator of your choice, which shouldn't be hard to find with a quick Google search. The site even provides a tool for compiling that ROM as a JPEG so you can upload it with your iOS 6 device through Safari. Once that's loaded, you'll be able to do anything you could do with the real calculator, using the exact same interface. The attention to detail is extraordinary. From the pixels on screen to the placement of the buttons, everything is how you remember it. You can even write or run programs within the interface, and even export or import them if you create a free account.
Nostalgia aside, this could be a great tool for helping your kids with maths homework, or just for brushing up on long-forgotten maths skills.