Embedded Tweet feature allows websites to generate copy and paste-able HTML markup to render a Tweet on any third-party website.
https://dev.twitter.com/docs/embedded-tweets
Widgets
Get access to data on 235,158 websites that are Twitter Embedded Tweets Customers. We know of 35,269 live websites using Twitter Embedded Tweets and an additional 199,889 sites that used Twitter Embedded Tweets historically and 23,750 websites in the United States.
Get a list of 235,158 websites using Twitter Embedded Tweets which includes location information, hosting data, contact details, 35,269 currently live websites and an additional 601,766 domains that redirect to sites in this list. 199,889 sites that used this technology previously and 23,750 websites in the United States currently using Twitter Embedded Tweets.