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.
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
draw.io - Online diagramming application
Balsamiq Cloud - Fast, approachable, collaborative wireframing
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.
Justinmind - Define awesome experiences through wireframes & prototypes.