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http://www.housingmaps.com/

This Google Maps visualize where to buy houses or rent rooms around US

Beautiful toy. :-)

Via lifehack.org

2. Amazon's concordance

Here it is, for Pinker's "The Language Instinct". Useful, if you have no idea what a book is about. :-)

3. Literature Map

Enter a writer's name and see what other authors you will love.

(Explanations on the site are less romantic, though: "The closer two writers are, the more likely someone will like both of them.")

I entered "Douglas Hofstadter" and got a two-dimensional list which included Christopher Alexander, Benoit Mandelbrot, Stephen Wolfram, Daniel Denett, Donald Knuth, Richard Dawkins etc. Not bad.