ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
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.
PowerMockup - Mockup & Wireframe toolkit for PowerPoint
draw.io - Online diagramming application
Wireframe.cc - Sweet and simple online wireframing 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.