Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Invision - Prototyping and collaboration for design teams
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Zeplin - Collaboration app for UI designers & frontend developers
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.