Canorus is a graphical music score editor written for the qt4 toolkit. it is a sequel of the well-known music score editor for linux, noteedit. canorus uses lilypond for music engraving to achieve a very high quality in the resulting documents.
canorus offers a number of import and export filters for lilypond, musicxml, midi, noteedit and others.
In its present state, I feel it's missing a number of features Denemo covers. The only real reason I can see right now to use Canorus over Denemo is the friendlier interface.