www.google.com
www.math.com

Both work extremely well for converting numbers, calculating and solving algebraic equations. I find them very handy when I lack a calculator or the patience to do all the calculations by hand when programming.

Try typing in something like (0xABC volt / 0b1101 ohms)/0o42 Hz in google. Lovely, isn't it?

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram