Wireframe.cc - Sweet and simple online wireframing 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.
WireframeSketcher - WireframeSketcher is a wireframing tool for creating wireframes, mockups and prototypes for desktop, web and mobile apps. It's a desktop app for Mac, Windows and Linux, and a plug-in for any Eclipse IDE.
draw.io - Online diagramming application
ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping 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.