TI Math Explorer (90's version.) I used to use a TI-89, but I sold it. Could kick myself for doing so. On the Explorer, I like its ability to do Fraction to Decimal conversions and Exponents...I don't really use a lot of Mathematics in my daily life, except for DIY projects, which it is very useful. I always had scientific calculators around me growing up, as my mom is a Geometry teacher. She would always bring home ones that students gave up or to help her grade papers.
A few months ago, I saw a calculator at the Dollar General for $5. It was a knock off of the TI-30XIIS (90's version), and had the same keypad layout and firmware. Couldn't pass up that deal.
As for the TI-89, I used it almost as a PDA. I was also trying to create a program for generate pressure/temperature charts for any given type of refrigerant. The science department at my high school used all sorts of accessories for their TI graphing calculators, such as thermometers and other types of instruments. Now 6 years out of there and all students and faculty use iPads for everything.