Wolfram Alpha is the power of Mathematica via the internet (for free). Offering many dynamic options and 3D graphing, this is a powerful tool for math teachers and students.
See http://blog.stephenwolfram.com/2011/08/wolframalpha-comes-alive-with-cdf/
See http://www.wolframalpha.com to try it out.
More details at: http://www.wolframalpha.com