Wolfram|Alpha - Computational Knowledge Engine

Wolfram|Alpha aims to bring expert-level knowledge and capabilities to the broadest possible range of people - spanning all professions and education levels.

Their service has helped me a lot throughout my time at the university. You can use it at Work, Home, School and Play. One of the best examples for leisure time uses of this website is plot cool images with equations.

Enter this Formula:
PolarPlot[(1 + 0.9 Cos[8 t]) (1 + 0.1 Cos[24 t]) (0.9 + 0.05 Cos[200 t]) (1 + Sin[t]), {t, -Pi, Pi}]
into Wolfram|Alpha and find out for yourselves.

And heres the output of the above equation:

via: Wolfram|Alpha