Wolfram Alpha: Computing the World's Knowledge
Wolfram Alpha also makes it extremely easy to convert between units:
From there, all kinds of useful calculations can take place, letting you really explore the breadth of human knowledge.
We hope you'll enjoy this thing as much as we have! If you manage to compute any amazing knowledge of your own, feel free to send it to email@example.com so we can all get some more cheap laughs at science beyond our understanding.