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
Pencil - Pencil is an animation/drawing software for Mac OS X, Windows, and Linux.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Zeplin - Collaboration app for UI designers & frontend developers