Axure - The most powerful way to plan, prototype and hand off to developers, all without code. Download a free trial and see why professionals choose Axure RP 9.
ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Balsamiq Mockups - Using Balsamiq Mockups feels like you are drawing, but its digital, so you can tweak and rearrange...
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Tabler - Admin panel made simple