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 217,616 websites that are Twitter Embedded Tweets Customers. We know of 79,898 live websites using Twitter Embedded Tweets and an additional 137,718 sites that used Twitter Embedded Tweets historically and 34,139 websites in the United States.
Get a list of 217,616 websites using Twitter Embedded Tweets which includes location information, hosting data, contact details, 79,898 currently live websites and 137,718 sites that used this technology previously and 34,139 websites in the United States currently using Twitter Embedded Tweets.