The OAuth protocol defines a common data structure for communicating between websites.

This relatively new format is increasing in popularity and used in many different sites including Facebook and Google. Microsoft has even added support for it in its latest WCF RIA services. (See my article with silverlight links below).

Read the full spec at IETF or try out some sample code.

References:
IETF, http://tools.ietf.org/html/rfc5849
OAuth.net, http://oauth.net
Quick Silverlight References, https://ronniediaz.com/2011/01/17/quick-silverlight-references/