Wolfram Alpha – The World’s famous search web site since 2009 (18/5).
It is a computational knowledge engine. Each time you type in the question, the answer will calculate(display) for you.
So, why need to sign up? You can enjoy(<—- copy from the confirmation E-mail)
1) step-by-step solutions (three times a day , get guided help with math problems)
2)Personal Analytics (analyze your Facebook profile!)
3) History and Favourites
4) Preferences (set your preferred unit of measurement, currency, etc)
5) Copyable plaintext