Wolfram Alpha is a math lover's dream. I've used Wolfram alpha to do everything from help me derive or check equations I've gotten published to helping kids on their math homework.
But that's not all it does. Check out their super cool example page and click around, you'll be amazed at everything Wolfram can do: from telling you what constellations you can see tonight, to telling you the nutrition info for 2 fig newtons, to calculating the flow around an airfoil, even comparing football team statistics: it seems there isn't anything Wolfram Alpha Can't do.
I loved Wolfram Alpha so much, i even got their smartphone app... It's great when i'm helping a student and need to remember something like the chain rule. It's useful if you're checking an answer to a math problem to click the "show steps" at the bottom. Other cool things to try are type in your name, ask it the speed of an unladen swallow, look up (and compare) stocks, and to solve systems of equations.
What are some of your favorite ways to use wolfram alpha?