Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
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.
Balsamiq Cloud - Fast, approachable, collaborative wireframing
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.
Justinmind - Define awesome experiences through wireframes & prototypes.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project