EPUB is the favorite format for e-books (ignoring Amazon, which like to be incompatible so it can lock users in). EpubCheck is the open-source industry standard for validating EPUB files. If you’re an author creating your own e-book files, you should run them against EpubCheck before releasing them. It’ll make hosting sites happier, since they’ll probably run it themselves and will like your book better if it passes. A book that passes EpubCheck will also give you fewer headaches with readers complaining it doesn’t work on their reader.
Version 4.0 was just released this month. It’s open source Java, available on GitHub. Several Web services let you check books online with it, but most of them haven’t updated to 4.0 yet. You can still use them.
Its new features include validation of EPUB 3.0.1, better internationalization, and more thorough warnings.