Wireframe.cc - Sweet and simple online wireframing tool.
Invision - Prototyping and collaboration for design teams
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.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project