Sigil

Program: Sigil

License: GPLv3

Description: "Sigil is a multi-platform WYSIWYG ebook editor. It is designed to edit books in ePub format." (From website)

Website: https://code.google.com/p/sigil/

Other: Sigil is able to run from portable media but requires installation on the host machine first, then copying the folder over. Hoping for an official PortableApps.com version that is fully contained.