As an engineering student, the TI-89 calculator is a necessity. It is the standard to base other engineering and advanced mathematics calculators. This calculator can do anything, even solving algebraic equations, derivatives, and antiderivatives. However, with so much functionality you'll have a tough time remembering the commands for everything. Each button can also do three tasks so the calculator includes two shift-like buttons, but it can be hard to read what each button does depending on the color of your calculator.
Aside from the math-related functions, you can even download a host of other programs into the calculator. Spreadsheets, calendars, whatever you want! It can be your own personal PDA if you need one and already carry the TI-89 around.
So, if you're looking for a calculator that virtually does anything and you are willing to go through the steep learning curve finding all of its functions, this is the calculator for you!