Texas Instruments TI-89 Titanium Graphing Calculator
TI's Motorola 68000-based TI-89 provides a built-in computer algebra system akin to computer software such as Mathematica and Maple. It can simplify mathematical expressions and perform partial fraction decomposition, useful for some intermediate calculus classes. Basic algebra, systems of equations, limits, derivatives, and integrals are also possible and can be performed symbolically. It can plot most anything including 2D, parametric, polar, and differential equations. Unfortunately it is too powerful to be allowed on the ACT exam, although it works for the SAT and APCalc. It's a little more expensive than the others and if you're not past the ACT you probably don't need a calculator this advanced. The TI-89 does support USB for downloading programs.