spatialreference.org is a website by Howard Butler, Christopher Schmidt, Dane Springmeyer, and Josh Livni that helps assist others in their understanding, recording, and usage of spatial reference systems.
While this site is coincidentally somewhat educational and fun to surf around in :), its real purpose is to allow you to use URL/URI-based references to spatial reference systems. You might think of it as a neogeo approach to a very oldgeo problem -- universally describing coordinate systems. However, rather than attempting to come up with yet another description and encoding scheme that few would use and would incompletely solve the problem, we've taken a slightly different approach. Instead, the expectation of spatialreference.org is to attempt to provide spatial reference descriptions in as many formats as possible, a sort of web-based GDAL for spatial references (by the way, GDAL and Proj.4 are the backbone of this website).
This approach allows you to refer to a URL that can produce a spatial reference in a format that your software can use.
There are currently three distinct catalogs of spatial reference systems on this website -- EPSG, ESRI, and User-defined (called sr-org). Others, like Oracle and ICU should be forthcoming.