Extension:TwitterCards

Description for Twitter
Twitter describes this feature like this:

Twitter cards make it possible for you to attach media experiences to Tweets that link to your content. Simply add a few lines of HTML to your webpages, and users who Tweet links to your content will have a "card" added to the Tweet that’s visible to all of their followers. It would be really nice for articles, images, videos - everything linked from Wikimedia on Twitter could have a nice little preview on Twitter. The documentation is at TwitterCards - basically it involves adding some meta tags around existing content server-side."

Resources

 * Feature request on Bugzilla
 * GitHub repository - work in progress
 * Twitter tool to Preview a Twitter Card