Sigil is a multi-platform WYSIWYG ebook editor. It is designed to edit books in ePub format.

Features:

Free and open source software under GPLv3
Multi-platform: runs on Windows, Linux and Mac
Full Unicode support: everything you see in Sigil is in UTF-16
Full EPUB spec support
WYSIWYG editing
Multiple Views: Book View, Code View and Split View
Metadata editor with full support for all possible metadata entries (more than 200) with full descriptions for each
Table Of Contents editor
Multi-level TOC support
Book View fully supports the display of any XHTML document possible under the OPS spec
SVG support
Basic XPGT support
Advanced automatic conversion of all imported documents to Unicode
Currently imports TXT, HTML and EPUB files; more will be added with time
Embedded HTML Tidy; all imported documents are thoroughly cleaned; changing views cleans the document so no matter how much you screw up your code, it will fix it (usually)
An actually usable user interface
Native C++ application
Bugs :)
And a lot more...