Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Atomic - The fastest way to design beautiful interactions
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Dotgrid - A simple vector illustration tool.
Invision - Prototyping and collaboration for design teams
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.