Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Balsamiq Cloud - Fast, approachable, collaborative wireframing
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Plectica - Easily diagram anything, together
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.