Although on the homepage they say denemo is alive the last version is still from 2005.
Perhaps this project here will do a fine job in the next years (not very useful yet):
The main developers of Noteedit (which is linux only) have moved to this project.
Without a real gui but with very helpful wizards I think Jedit is a good choice as a Lilypond editor:
Because there is a free addon (which can also be downloaded or updated from within the editor):
See the videos on the page.
Lilypondtool uses an internal pdf viewer which allows to use the point and click feature of the Lilypond generated files. So you can click on a note and immediately get to the right spot in the lilypond file. A feature that doesn't really work with Adobe viewer on windows. At least for me it doesn't.