Mobipocket Creator (Windows)
allows to create ebooks by adding HTML files and images, create personalized cover page, add metadata, import Word documents, texts and PDF, browser preview and many other features
Writer2ePub (multi OS)
LibreOffice and OpenOffice extension that allows to save documents in EPUB format
iBooks Author (Mac)
most popular Mac software for ebooks creation, it allows also to publish them in iBooks store
Papyrus (online)
doesn’t require software to be installed, it’s an online editor that allows to create, save (in PDF, EPUB, Kindle format) and publish ebooks