I second ipe, used it together with LaTeX for my theses. It looks great, gives you many options while being an easy-to-use WYSIWYG tool and integrates with LaTe

I second ipe, used it together with LaTeX for my theses. It looks great, gives you many options while being an easy-to-use WYSIWYG tool and integrates with LaTeX typesetting (you can typeset in the diagram). The PDF export makes it also usable in other contexts.