There's a new resource online called Wolfram|Alpha (http://www.wolframalpha.com/). It looks like a search engine, but really it's supposed to be a direct source of information. It also does math for you.
There are tons of things you can do with it, as demonstrated by this awesome demo video. I highly recommend it.
Here is part 2:
You can ask for the GDP of France divided by that of italy, word frequencies, nutritional information, geographical information, etc.
Wolfram|Alpha takes its information from many data sources. When I saw a talk by Stephen Wolfram, I asked him about citing it. Can we get the database from which the site got the information so we can cite it?
His answer was very interesting. He wants the site to be a source itself, much like an encyclopedia. He also said that often, even though the site uses databases to get its answers, the actual number you see is probably not in any particular one. It averages, it weights, and comes up with its answer. Just like you'd trust an encyclopedia, you are asked to trust the AI behind the site.
The webpage does not suggest how, exactly, to cite it, but I recommend the following form: Let's say you wanted to say that running 30 minutes burns about 352 Calories (Wolfram|Alpha, 2009). The citation has the exact date retrieved (populations of countries, for example, will vary daily, I'd imagine), and the query used.
Unfortunately, one cannot replicate the search and expect to get the same number, because Wolfram|Alpha gets the best information it can at the moment of the search. A useful tool.
Wolfram|Alpha (2009). retrieved August 24, 2009, from http://www.wolframalpha.com/ with query [running 30 minutes].
Pictured: relative sizes of astronomical objects. Some of the data for this image was taken from Wolfram|Alpha.