Galaxy Zoo is the citizen science project that allows users to identify and classify galaxies according to their appearance in images taken from the Sloan Digital Sky Survey 2 (and some new ones from the Hubble Space Telescope). One thing this is particularly useful for is identifying by eye certain morphologies of galaxy that would take ordinary observers years to pick from the morass of targets out there.
Bill Keel and colleagues have built up, for example, a collection of 2187 pairs of overlapping galaxies (ie two galaxies that are far apart, but from our perspective, look to be one directly in front of another). These 2187 represent all the pairs dug out of the Sloan survey (the Hubble data has now taken over completely), but they haven’t been fully cross-checked to remove double sightings. When this has been completed, there will be an instant catalogue of overlapping galaxies, including the type of overlap (an X-type would have two edge on galaxies overlapping, a Q type has one edge on and one face on etc). He explains here the scientific use of overlapping galaxies and why he is interested in them.