I have used this app only once when Amazon was refusing my book in Calibre's mobi format and it saved my life. :-)
I opened with Sigil the epub generated by Calibre and generated the TOC (Tools/Toc/Generate at that time) then I opened the edited epub in Kindle Previewer or kindlegen and converted to mobi.
So 5 stars for this invaluable tool!
8.01 is out at http://sigildev.blogspot.it
A really nice and great application to create or edit epub.
My only complaints with these last versions (8.0 and 8.01) is that you can not paste text in the Replace field using key shortcut anymore.
It is not a tragedy, but it's not so comfortable activate the control-click menu every time you want paste the replace string in Search and Replace…
So before to upgrade take also a backup copy of previous version if it works for you.
Great software, unfortunately no longer under active development. See: http://sigildev.blogspot.com
I am still using it in Mavericks. I use Calibre also, which is constantly improving as an ebook editor.
Sigil 0.6 is out for some time and this version allows to switch HTMLTidy of so (at last!) it is possible to insert html5 tags (like video and audio).
Sigil is better for epub creation (by hand) than Calibre because it does not insert it's own css.
At the moment this is the best software available: I have tried Jutoh, but development seems to have stopped (and the interface is ...not standard - to say the least!) and conversion utilities like eCub (and Calibre), Legend Maker, eBookBinder, MePub and a lot of Python scripts (I even wrote one myself) but none is as easy to use as Sigil.
So: at this moment this is the best option available - even as it is slow on saving projects because it uses an already packed epub file format. Recommended!
Just a quick update for anyone still around. The Sigil team are working hard toward the release of 0.6.0, which should have interface improvements as well as many upgrades to code. If you're looking for a pure Mac interface, don't bother; Sigil is intended to be crossplatform so that it looks an operates the same in all OSes. That being said, I've seen glimpses of the new interface, and I think some complaints should be layed to rest in that area.
If you want to create epub ebooks, Sigil is a very good tool: you'll get state of the art, clean epub files.
Of course, it looks like a Windows software: anti ergonomic and for geeks only.
But hey, it works, and it's free.
This listing is still outdated, even though the program is in active development:
Last PowerPC Compatible Version
Version 0.3.4 is not slow on my iMac. It is the best app to build I have tested these few days. Making very good and standard compliant code. It is useful to use it with a text editor as helper, like BBEdit or TextWrangler for instance. BBEdit is preferred because it can deliver and validate good code andTextWrangler is the light version of the soft. I'd like a better integration of Sigil with Mac OS guidelines but it's still a good app and it can progress a lot.
A very basic ePub editor. Extremely slow, however, and completely unresponsive (color wheel) during major replace operations. (And replacing misrepresented characters and typos is my main use of this utility.) OK, it is still v. 0.1.8, but this is not a very helpful one. Still has to get much faster and more responsive.