Validators

Feed Validator for Atom and RSS
This is a validator for syndicated feeds. It works with RSS 0.90, 0.91, 0.92, 0.93, 0.94, 1.0, and 2.0. It also validates Atom feeds.
http://www.feedvalidator.org/

Feed Validator for RSS and Atom
Provides schema-based online validation for Atom and RSS feeds and a large testsuite for developers of similar tools.
http://www.validome.org/rss-atom/

Feedvalidator-users Email List
Questions, bugs, patches, and general discussion of FeedValidator.org's Atom and RSS feed processing service.
http://lists.sourceforge.net/lists/listinfo/feedvalidator-users

Redland RSS 1.0 Validator and Viewer
Enter the address of an RSS 1.0 feed into the form, and it will be validated and formatted for display.
http://librdf.org/rss/

RSS1.0 Validator (ldodds.com)
An experimental online RSS1.0 Validator
http://www.ldodds.com/rss_validator/1.0/validator.html

Userland RSS validator
Userland Software's online validator for RSS 0.9x
http://rss.scripting.com/

W3C RDF Validation Service
A full RDF Validation Service - for use with any RDF, not just RSS 1.0
http://www.w3.org/RDF/Validator/

Will the real RSS validator please stand up? [dive into mark]
Elaborates on how the RSS community is in disagreement about the exact format for RSS feeds and individual entries. <small>(June 26, 2003)</small>
http://diveintomark.org/archives/2003/06/26/will_the_real_rss_validator_please_stand_up