Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Atomic - The fastest way to design beautiful interactions
Invision - Prototyping and collaboration for design teams
Dotgrid - A simple vector illustration tool.
Zeplin - Collaboration app for UI designers & frontend developers
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project