Writer2ePub is a free extension for OpenOffice/LibreOffice/NeoOffice that creates EPUB files from .odt files and all other text files readable by *Office suites.
It is not a file conversion utility, but a real EPUB development tool for Publishers and Self-Publishers. The goal of W2E is to create a ready-to-publish EPUB that needs no further editing at XHTML-CSS-XML level. Anyway, the generated code is very clear and easy-readable.
Metadata and other settings are editable and stored inside the .odt document, making simple the correction and interchange of the source file, saving time and cost for eBook development.
Version 1.x is downloadable from W2E site:
http://writer2epub.it/en
A version 2 is planned soon, with many improvements including EPUB3 output, editing window, source cleaning and correction tools and more.