If it’s computable knowledge, WolframAlpha wants to know about it. This knowledge engine is a bit like Google on steroids. It was developed by Wolfram Research, the makers of Mathematica, which is one of the drivers of the knowledge engine. Have a look at this Screencast where Stephen Wolfram gives you a “quick introduction” to WolframAlpha and the types of queries it can handle.
Here’s an article on ZDNet that dissects WolframAlpha from more of a geek perspective. Wolfram Research is building a knowledge base of computable data that ranges from musical scales complete with audio playback to complex formulae and graph displays on the physical properties of matter. Ask WolframAlpha about the melting point of copper, for example, and apart from computing the answer in Celsius (1084.62 deg C) – or Fahrenheit or Kelvin if you prefer – it will also offer the fact that this is 190 deg. hotter than a large log fire, and offer spectral radiance graphs based on frequency and wavelength over a range of 6000 degrees.
You can download a WolframAlpha add-on for IE 6+, Firefox 2 or 3, or as an iGoogle Gadget, from the WA downloads page.