I'll start off with something simple here. A tool that I have found incredibly useful when trying to check my work is WolframAlpha (http://www.wolframalpha.com/). I can't tell you how many students I've had tell me that it's been a life-changer. Just type in an equation, and it will give you graphs, solutions, alternate forms, and much more. This is especially useful for higher level math and graphical analysis.