Please be nice append your tools to the end of each list. Thanks.
RSS2 to SAM (formerly (n)ECHO) converter (XSLT-MSXML 4.0)
RSS2Necho (converts any RSS to necho 0.0)
Patch to add Echo support to the Straw aggregator http://www.licquia.org/code/straw-0.18.1-necho-0.1-patch.diff
Driver for Echo, in Radio Userland aggregator http://www.brainoff.com/necho/
ATØM feed - XSLT application to transform Input (blogroll OPML) files with ATØM, RRS2 or RSS1 feed links to an XHTML+SMIL+VML composition User interactions: input of OPML file, Categories, Sorting etc parameters. (for IE6 users with MSXML4) http://cybarber.ath.cx/ATOMRSSfeedcombiner.html
Now online: Readback version with MS Agent(character Claude) and Living Actor(Actor Claude) using SAPI4 British english TTS3000. http://cybarber.ath.cx/ATOMRSSfeedcombinerReadback.html - [Cybarber}
clevercactus beta 2 includes preliminary Atom support
Drop-in replacement for nntp//rss 0.3 to support necho 0.1 and atom 0.2 feeds http://www.isolani.co.uk/blog/semanticweb/EarlyHackingInNecho
NewsGator supports atom 0.2 feeds.
Feed Validator http://feedvalidator.org/
Mark Pilgrim's ultra-liberal feed parser supports necho 0.1 http://diveintomark.org/projects/feed_parser/
[DeveloperDude] kbcafe.atom is a C# toolkit/class for creating and consuming Atom syntax (not the API yet, I'll do that later). Please feel free to give feedback at the specified URL. By the way, it's less than 200 lines of code (mostly empty comments). Tell me that C# isn't cool!
[JeremyAshcraft] phAnTOM The phAnTOM parser is a PHP class for parsing Atom 0.2 feeds. Still in its early stages as it can only handle <content> elements that are escaped XML/HTML or text. Part of the Simplog project to add Atom support into the News Reader. Download it, dissect it, improve it. I'm going to need help to make this thing work 100% the way it is supposed to.
Bikini - a new sourceforge project
update: looks like someone was working on a Perl module
[UrsGehrig] There is an Atom 0.1 feed generator for the blog tool Wordpress (b2 resp.) on http://www.phaedo.cx/archives/php/i_created_a_atom_01_feed_generator_for_wordpress_today.html
AtomiXXE is a configuration for editing Atom files in the XMLmind XML Editor.