This is worth re-posting…the feed validator has moved to http://feedvalidator.org.  There are other validators out there, but this is the only one really worth using, IMHO.

The Feed Validator, previously located at feeds.archive.org/validator/, now has its own domain: feedvalidator.org. If you have any scripts, templates, or applications that point to the Feed Validator, now would be a good time to update them. [dive into mark]