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.

Until Later,

Ashley