webvatar
Webvatar was a service that provided direct URLs for <img> tags when you had the domain, or website address, of the user.
Thus, by using <img src="http://webvatar.com/:website"> (with or without the scheme -- http:// or https://) you can shorten your need for servers and microformats2 parsing and still get people's pictures.
Webvatar was rewritten in Go from its original Python code and is now between 5 and 10 times faster than before (it also uses Cloudflare cache and an internal Redis cache helper to increase speed even more). It has support for microformats2 u-photo and rel="icon" and a lot of nice fallbacks (mostly borrowed from other public services) for when no image is found.
As of September 3, 2015, Webvatar supported major silos like Facebook, Instagram, Twitter, GitHub, Google+ and Trello URLs, so webvatar.com/https://twitter.com/fulano and variations would work. See examples below.
Source Code
The Webvatar source code is available on Github. Feel free to fork it and submit pull requests, or just report bugs or suggest things.