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.
Axure RP - The prototyping and specification software used by 87% of the Fortune 100. Beyond mockups, design interactive prototypes for business solutions, websites, and applications.
Saola Animate - Easily create engaging HTML5 animations and interactive web content without coding skills
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.