http://code.google.com/p/feedvalidator/

This is the validator that powers the Atom validator at http://validator.w3.org/feed/

Perhaps it can be modified for OPDS-specific elements...

-raj

While there may be some value in re-using the existing code, when I
reviewed it I noted that it was evolved from RSS validation and
tremendously focused on blog feeds rather than generic Atom.

Keith

For what it's worth, every time a new customer wants me to see what's wrong
with their OPDS feed, I start with that validator.  It throws a heck of a
lot of warnings that are not actually valid error conditions in OPDS, but I
almost always find the output useful.  It might take some stripping off of
the blog specific code like Keith says, but I suspect it would be a valuable
starting point as well...