The Sloan Digital Sky Survey (SDSS) is a project dedicated to mapping the universe. Over eight years of operations, it has obtained deep, multi-color images covering over a quarter of the sky and created 3-dimensional maps containing more than 930,000 galaxies and more than 120,000 quasars.
SDSS provides a SOAP API that enables users to perform a cone search of the night sky. A cone search is a (relatively) simple request for astronomical information, retrieving whatever is available for a given position in the sky and a given radius about that position. Users must describe the area of sky on which they want information in terms of RA (right-ascension), DEC (declination), and SR (search radius).