Microdata for adding products to Google Shopping.
http://support.google.com/merchants/answer/6069143?hl=en-AU
Widgets · eCommerce
Get access to data on 3,477,231 websites that are Microdata for Google Shopping Customers. We know of 986,268 live websites using Microdata for Google Shopping and an additional 2,490,963 sites that used Microdata for Google Shopping historically and 490,711 websites in the United States.
Get a list of 3,477,231 websites using Microdata for Google Shopping which includes location information, hosting data, contact details, 986,268 currently live websites and an additional 617,792 domains that redirect to sites in this list. 2,490,963 sites that used this technology previously and 490,711 websites in the United States currently using Microdata for Google Shopping.