Sigil

Sigil is an open-source eBook editor that works under the GPLv3.

Editing with Sigil follows the WYSIWYG (what you see is what you get) principle and works almost identically to other text editing programs like Microsoft Office.

Currently, Sigil can import files in TXT, HTML, and EPUB formats, and export in EPUB and SGF (Sigil's native format).

The program is very easy to use, and even though it is still in early development, includes many interesting features for eBook owners.