TI-89 Titanium

I originally bought this calculator for high school AP Calculus, and was able to take advantage of its symbolic differential equations and indefinite integration capabilities to check my work. I went on to be a chemical engineering major and quickly discovered its capabilities extended far past that, and used it, for all intents and purposes, as a portable, barebones MATLAB. It does single variable and basic multivar calculus problems explicitly, as well as matrix manipulation, a dedicated equation solver, a built-in unit converter, extensive 2D and 3D graphing capabilities, a stat toolbox that honestly rivals EXCEL in everything except interface, and a dedicated community (www.ti-calc.org) of independent app writers that fills any niches and voids your work may require (such as databases of physics equations and periodic tables).