Really Simple Discovery is a way to help client software find the services needed to read, edit, or "work with" weblogging software.
http://cyber.law.harvard.edu/blogs/gems/tech/rsd.html
Syndication Techniques
Get access to data on 42,287,216 websites that are Really Simple Discovery Customers. We know of 2,000,003 live websites using Really Simple Discovery and an additional 40,287,213 sites that used Really Simple Discovery historically and 1,128,634 websites in the United States.
Get a list of 42,287,216 websites using Really Simple Discovery which includes location information, hosting data, contact details, 2,000,003 currently live websites and an additional 1,557,253 domains that redirect to sites in this list. 40,287,213 sites that used this technology previously and 1,128,634 websites in the United States currently using Really Simple Discovery.