Today marks the fifth anniversary of the official introduction of Google rich snippets, announced in a Webmaster Central blog post of May 12, 2009.
Google had been experimenting with "enhanced listings" as far back as December 2008, with review and product rich snippets being spotted in results for Yelp, Citysearch, CNET, TripAdvisor and Download.com. The May 2009 announcement provided these enhanced listings with an official name, and explained how these results were generated.
The announcement post extolled the virtue of giving "users convenient summary information about their search results at a glance" through rich snippets, and provided information about the markup formats that were used to generate rich snippets – intially microformats and RDFa.
The number of rich snippet types displayed in Google has grown steadily over the past five years. I've compiled a list of these rich snippets, along with a rendering of the example snippet that accompanied each Google announcement. This inventory may not be wholly comprehensive, but I believe you'll find the most important rich snippet types listed.
Review/ratings rich snippets and people rich snippets
May 12, 2009
"We are currently supporting data about reviews and people," said Google in their announcement
A follow-up post published on April 26, 2010 announced that rich snippets would be supported internationally
Events rich snippets
January 22, 2010
Recipe rich snippets
April 13, 2010
Authorship rich snippets
June 7, 2011
This post announces support for authorship markup rather than author rich snippets explicitly
Music rich snippets
August 18, 2011
Application rich snippets
September 14, 2011
August 6, 2013
Many rich snippets, like breadcrumbs or video, weren't officially announced but made their appearance in the search results over time. And are the innumerable stand-out results in Google "rich snippets," "search verticals," or simply evolved search results? To-may-to, to-mat-oh, in my opinion.
And there's been many technical tweaks and infrastructure changes at Google in support of rich snippets, even if these haven't resulted in the generation of new rich snippets or changes to the appearance of existing rich snippets. Milestones include the launch of the Rich Snippets Testing Tool (October 2009, renamed the Structured Data Testing Tool in September 2012), support for microdata (March 2010), and – of course – the launch of schema.org (June 2011).
The introduction of rich snippets into Google search results marked the beginning of the end of "ten blue links." Rich snippets both conceptually and visually paved the way for Knowledge Graph and even, perhaps, signaled the point at which Google ceased to be a simple provider of links to other websites and set its sights on becoming a purveyor of answers in its own right.