ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
Wireframe.cc - Sweet and simple online wireframing tool.
PowerMockup - Mockup & Wireframe toolkit for PowerPoint
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.
mockingbird - Mockingbird helps you you create and share clickable wireframes. Use it to make mockups of your website or application in minutes.
Invision - Prototyping and collaboration for design teams