Sigil 0.9.9

"Sigil 0.6 � A worthy eBook program"

Pros: � Can create (high-quality) ePub files � Can validate the file with the in-built validator and the W3C CSS validator � Can open HTML and TXT files as well � Code view � Open source

Cons: � Difficult learning curve � Finding Linux binaries can prove a pain � No MOBI output (use Calibre for conversion)

Other Thoughts: Sigil is a powerful tool that has a wide variety of features, including spell-checking, the ability to view (and edit) the HTML code, support for ePub and a nice GUI.
However, you will need at least a basic understanding of HTML and CSS to make full use of it. It also requires conversion for Amazon kindle, and you have to look for a repository for your distro. Still, it's good overall, and is progressing rapidly.

The only other thing is that it is a FORMATTING program, so actually WRITING on it isn't a very pleasant experience.