Okay, I do not normally go on talking about this site or that site that I think is cool because ... well, because I do not normally want to be thought of as a person with a web page that just promotes other web pages. I mean, really.
But Wolfram Alpha is so flippen cool. It is not a search engine, it is a knowledge engine. (Yeah, it says that on the page, but I can't think of a better way to put it.) There is so much cool stuff you can do with the site.
Mathematics and unit conversion are what I use it for the most, but it just KNOWS SO MUCH! (It is much awesomeness.)
Check it out. Play with it. Challenge it!