ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
PowerMockup - Mockup & Wireframe toolkit for PowerPoint
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
Wireframe.cc - Sweet and simple online wireframing tool.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project