Handy, yes, and also incredibly powerful. This app manages a range of equations, including situations in which you need to define your own variables and functions. The handy part? Commonsense functionality, like asking you in plain English if you want to solve equations with a decimal or a variable.
WIRED Plots expressions on a graph. Converts units and currencies with up-to-the-minute exchange rates from Yahoo Finance.
TIRED Can't replace your graphing calculator completely.