epubcheck (the "official" java tool)
http://code.google.com/p/epubcheck/
EPUB Validator (online, idpf)
http://validator.idpf.org/
ePub Check Batch (Windows)
(Windows Batch file wrapper for Drag&Drop)
https://www.mobileread.com/forums/sho...d.php?t=201764
epubcheck for Mac OS X (AppleScript wrapper by pdurrant)
https://www.mobileread.com/forums/showthread.php?t=55576
FlightCrew (standalone version, also built in Sigil) (MacOS, Windows, Linux)
http://code.google.com/p/flightcrew/
Pagina EPUB-Checker (Windows/Mac/Linux)
http://www.pagina-online.de/software/epub-checker/
There are other tools out there (some commercial, but IMHO they're not any better than those listed above, just because they have a price tag!) - generally, check the file dates, many, if not all of them are old and not maintained!
Regards,
Kim