draw.io - Online diagramming application
EazyCSS - No code CSS editor for any website.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Hoverify - All-in-one browser extension to improve your web dev experience.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Devsync - Visually edit CSS. Live-sync the result with your editor.