These days you don't need to waste your money on a scientific calculator thanks to the spread of mobile software that will do the same job for you but for free.
TouchCalc is a highly usable tool that transforms your iPhone into a scientific calculator. Not only does it provide you with vast amounts of computational power, but it is one of the most touch-friendly iPhone apps around. TouchCalc manages to cram a lot of function buttons onto the interface and yet still make them easy to select for even the thick-fingered user.
The great thing about TouchCalc is that not only is it a scientific calculator with all the usual arithmetic and trigonometric operations, but it also lets you perform binary Boolean queries, and includes a statistical calculator. As a result, TouchCalc gives you the power to create sample data and calculate mean, variance, deviation, and more.
One of the few negative things about TouchCalc is that it looks rather drab. Granted, it's supposed to be a serious app, but it would've been nice to see a funky design treatment in keeping with the coolness of the Apple device, rather than something that looks like an instrument from the 1980s.
That said, if you're a mathematics student or a scientist and you have an iPhone, TouchCalc should be one of the first apps you install.