RSS (Rich Site Summary)
RSS 2.0
This document represents the status of RSS as of the Fall of 2002, version 2.0.1. It incorporates all changes and additions, starting with the basic spec for RSS 0.91 and includes new features introduced in RSS 0.92 and RSS 0.94.
Content feeds with RSS 2.0 (IBM)

RSS 1.0
The RSS 1.0 specification was released on December 6, 2000.

RSS 0.93
This version was discussed but never deployed. [April 20, 2001]

RSS 0.92
This version is 0.91 with optional elements, designed by Dave Winer at UserLand. [December 25, 2000]

RSS 0.91
This version was designed by Netscape, spec written by Dan Libby. [July 10, 1999]

RSS 0.90
This version was designed by Netscape, for use with my.netscape.com, which also supported scriptingNews format. The only thing about it that was RDF was the header, otherwise it was plain garden-variety XML. [March 15, 1999]

Atom
Atom 1.0
Atom wiki
RFC4287 - The Atom Syndication Format

Feed's Resource
WebReference RSS Info page
RSS (file format)
Atom VS RSS!
Atom Feed
RSS Tutorial


Feed Tool
FEED Validator for RSS and Atom
userLand RSS validators