EPUB Checker

When exporting documents to EPUB format, it’s important to check them for errors — especially if you edited it after exporting. One tool that helps is EpubCheck, which can detect many types of errors in EPUB. OCF container structure, OPF and OPS mark-up, and internal reference consistency are also checked.

Get it at http://code.google.com/p/epubcheck/ or https://github.com/IDPF/epubcheck.