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...