OAuth

OAuth is 3rd party authorization protocol, very useful for web apps that utilize data about their users from other services like Facebook, Google, Twitter, Meetup, Github and so on.

Official site: http://oauth.net/

Versions
Many service providers adioted OAuth and support both v1.0a and v2, but the specific drafts supported of v2 vary. Google deprecated support for v1.0a for their APIs.

OAuth 1.0a
OAuth Version 1.0a is standardized as RFC5849.

OAuth2
Newer version, OAuth 2.0 drafts replace encryption layer with SSL, but complicates token handling introducing token expiration and imposing token lifetime management on developers.

Service Providers OAuth Docs

 * Facebook
 * Google
 * Twitter
 * LinkedIn
 * Meetup
 * Github
 * Etsy
 * Yahoo
 * ... add yours ...

Links

 * Towards OAuth2 Pastures - Meetup.com's use of OAuth2