Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
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
Zeplin - Collaboration app for UI designers & frontend developers
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Fluid UI - Fluid UI helps you prototype Android, iPhone, iPad or Windows 8 mobile apps with our custom libraries, or upload your own images for pixel perfect mobile