Argotic Syndication Framework
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Argotic Syndication Framework - Home
A powerful and extensible .NET web content syndication framework for RSS, Atom, OPML, APML, BlogML, RSD and more. The Argotic Syndication Framework is a Microsoft .NET class library framework that enables developers to easily consume and/or generate syndicated content from within their own applications. The framework makes the reading and writing syndicated content in common formats such as RSS, Atom, OPML, APML, BlogML, and RSD very easy while still remaining extensible enough to support common/custom extensions to the syndication publishing formats. The framework includes out-of-the-box implementations of the most commonly used syndication extensions, network clients for sending and receiving peer-to-peer notification protocol messages; as well as HTTP handlers, modules, services and controls that provide rich syndication functionality to ASP.NET developers.

XML.com: Atom Authentication
XML.com: Atom Authentication

A nice article with detailed explanation of the atom authentication protocol
WP-APP: Atom Publisher Protocol for WordPress
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WP-APP: Atom Publisher Protocol for WordPress
As the title says, this is an one file drop-in implemetation of ATOM for WordPress.
XML.com: Normalizing Syndicated Feed Content
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XML.com: Normalizing Syndicated Feed Content
So you want to write a program to read RSS and Atom syndicated feeds. Sounds simple enough. After all, RSS stands for "Really Simple Syndication" (or "Rich Site Summary", or "RDF Site Summary", or something), and Atom is just RSS with different tag names, right? Well, not exactly.
Google Data APIs Overview
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Google Data APIs Overview
The Google data APIs ("GData" for short) provide a simple standard protocol for reading and writing data on the web. GData combines common XML-based syndication formats (Atom and RSS) with a feed-publishing system based on the Atom publishing protocol, plus some extensions for handling queries.
Rss20 And Atom10 Compared
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Rss20AndAtom10Compared - Atom Wiki
People who generate syndication feeds have a choice of feed formats. As of mid-2005, the two most likely candidates will be RSS 2 and Atom 1.0. The purpose of this page is to summarize, as clearly and simply as possible, the differences between the RSS 2 and Atom 1.0 syndication languages.
Hypocrisy: RDF and the Atom API
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Hypocrisy: RDF and the Atom API
If the Atom / RSS discussion awakened your curiosity, it might be interesting to read this rant and comments underneath.