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
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
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 - Balsamiq. Rapid, effective and fun wireframing software.