I’ve been using a ti89, first the original release and then the titanium release, since I was in high school. It is a calculator for the hardest core of geeks.
Throughout engineering school–I’m working toward my Masters in Mechanical Engineering–I’ve used the programming app to code contours of cam surfaces as well as a host of matrix and kinetics/kinematics programs to help me through exams and to speed up homework problems. It’s a graphing calculator, which allows me to overlay 99 plots of different functions; I can turn on and off different plots to select which functions I’d like to compare.
The ti89 comes standard with a suite of calculus tools such as integration and derivatives, as well as many linear algebra operations. It also comes with support from a Texas Instruments-based free online app store, as well as a programming editor that allows the user to write his own code. There are also preloaded finance and electrical engineering programs.
I own three of these handy calculators: one for my desk at work, another for my desk at home and one for my laptop/school bag.