Google never fails to amaze me. Their site is not only the best search engine around, but it also includes additional useful functionality – maps, phone book, stock quotes, etc.
It now even has a built-in calculator.
Want to know the mass of the sun * pi as measured in stones? Pounds? Grams? Atomic Mass Units? How about pennyweight?
How about converting 3,737,844,653 to hexidecimal? Binary? Octal?
Very handy.