Mushrooms
feedvalidator.org feeds.archive.org feedvalidator.sourceforge.net atomenabled.org rss.scripting.com
And, coming soon, there will be one on localfeeds.com
Apparently, there is some controversy over scripting.com hosting the feedvalidator. From my perspective, the license clearly permits this usage. Futhermore, I encourage it. And if Dave or Andrew want to submit patches to provide an option to disable validation of Atom and/or RSS 1.0 on their installation, then I will see what I can do to integrate such patches into the system.
Meanwhile, the validator got its first bug report on sourceforge. The validator currently issues errors when it encounters relative urls is RSS feeds. Unless I hear any objections, I'll remove all such checks, messages, and test cases pending resolution of this issue.
I've submitted requests to create a CVS repository and mailing list on sourceforge. I'm told that these should be available shortly.