Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Invision - Prototyping and collaboration for design teams
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Atomic - The fastest way to design beautiful interactions
Pixelapse - Visual version control and collaboration workflow for open source and private design projects.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.