Invision - Prototyping and collaboration for design teams
PowerMockup - Mockup & Wireframe toolkit for PowerPoint
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Wireframe.cc - Sweet and simple online wireframing tool.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
mockingbird - Mockingbird helps you you create and share clickable wireframes. Use it to make mockups of your website or application in minutes.