There are many articles out there on how DNS prefetching works so I will keep it brief.
It works by adding a link to the head of your web page with a relationship of 'dns-prefetch' and the url of a domain which holds assets needed for the page to load. That could be a cdn for the site, a social plugin or anything else.
Without prefetching, a browser will render the page top to bottom and when it gets to an asset which needs to be loaded from another domain it performs a dns lookup to find the location of the host and then downloads the assets. By adding prefetching links we are letting the browser know that these domains will be used before the assets are requested. Browsers can then loook up the hosts whilst rendering the rest of the page, cutting out this step when the assets are needed leding to a shorter download time.
This process is non-blocking and will only be performed when possible. For example, on a small page requesting an asset from another site high in the head of the document, it's likely that the asset will be loaded before there is a chance to prefetch the dns.
The point of this article is to list domains used by common services for easy future reference.
Social Buttons
//twitter.com
//platform.twitter.com
//connect.facebook.net
//static.ak.facebook.com
//s-static.ak.facebook.com
//fbstatic-a.akamaihd.net
Google Plus
//apis.google.com
//ssl.gstatic.com
//oauth.googleusercontent.com
//accounts.google.com
//oauth.googleusercontent.com
//www.linkedin.com
//platform.linkedin.com
//static.licdn.com
Disqus
//disqus.com
//go.disqus.com
//a.disquscdn.com
//juggler.services.disqus.com
//referrer.disqus.com
You should also add your disqus subdomain
//subdomain.disqus.com
Google Analytics
//www.google-analytics.com
Google Code (jQuery etc)
//ajax.googleapis.com
Google Fonts
//fonts.googleapis.com
//fonts.gstatic.com
//themes.googleusercontent.com
Google Maps
//maps.gstatic.com
//maps.google.com
//maps.googleapis.com
//mt0.googleapis.com
//mt1.googleapis.com