A button that lets you "mention" another user on Twitter.
https://twitter.com/about/resources/buttons#mention
Widgets
Get access to data on 23,048 websites that are Twitter Mention Button Customers. We know of 6,301 live websites using Twitter Mention Button and an additional 16,747 sites that used Twitter Mention Button historically and 3,625 websites in the United States.
Get a list of 23,048 websites using Twitter Mention Button which includes location information, hosting data, contact details, 63,056 currently live websites which includes 56,755 domains that redirect to sites in the list. 16,747 sites that used this technology previously and 3,625 websites in the United States currently using Twitter Mention Button.