Many of my students and colleagues also have an iPhone or iPod. While there are many calculator options for these devices, I would like to recommend Scientific Graphing Calculator for $2.99, because it has useful mathematical references and makes graphing easy. It allows up to 4 equations to be drawn on top of each other, and it calculates points of intersection. However, I find it a bit clunky as a simple calculator.
When test time comes around, students will usually have to use a dedicated calculator. However, Scientific Graphing Calculator is cheap, convenient (as long as you always carry your iPhone/iPod), and surprisingly powerful. I prefer it for doing homework through pre-calculus.
For calculus I and above, I suggest MathStudio. It can take some time to learn, but its operations are plentiful and the 3D graphing is super.