LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Atomic - The fastest way to design beautiful interactions
draw.io - Online diagramming application
Dotgrid - A simple vector illustration tool.
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.
Invision - Prototyping and collaboration for design teams