Wireframe.cc - Sweet and simple online wireframing tool.
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.
ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
Saola Animate - Easily create engaging HTML5 animations and interactive web content without coding skills
Invision - Prototyping and collaboration for design teams
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project