Mobi Calculator FREE (176 KB), an excellent and easy-to-use calculator with math expressions, memory and history entries that can be edited for a recalculation without the need to enter them all over again. It comes with scientific functions and other useful features such as radix mode, time calculation and more.
RealCalc Scientific Calculator (296 KB), a scientific calculator that's fully featured and complete with unit conversions. Useful features include result history, digit grouping, 10 memories and more. Handy to use on a mobile device.
handyCalc Calculator (1.3 MB), powerful enough to solve equations with variables and functions, draw graphs for math expressions, and with unit and currency converters built-in, though it didn't respond well to a long expression when tried. (If the keypad doesn't work, please try the previous version from the developer's site.)