Microdata for adding products to Google Shopping.
http://support.google.com/merchants/answer/6069143?hl=en-AU
Widgets · eCommerce
Get access to data on 4,314,325 websites that are Microdata for Google Shopping Customers. We know of 1,778,276 live websites using Microdata for Google Shopping and an additional 2,536,049 sites that used Microdata for Google Shopping historically and 933,508 websites in the United States.
Get a list of 4,314,325 websites using Microdata for Google Shopping which includes location information, hosting data, contact details, 1,778,276 currently live websites and an additional 1,427,402 domains that redirect to sites in this list. 2,536,049 sites that used this technology previously and 933,508 websites in the United States currently using Microdata for Google Shopping.