Online Feed Validation from W3C
The Feed validator can validate RSS (0.90, 0.91, 0.92, 0.93, 0.94, 1.0, and 2.0) as well as Atom feeds. What is it validating these feeds against? Why the RSS 2.0 specs of course.
However, in addition to what the FeedValidator software already did, W3C had to (of course) make it a little better. They added in a SOAP web service, and are providing the online help to implement it in software (where it could be used for batch processing).