Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Platforma - Wireframe library of layouts and UI elements
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.