Invision - Prototyping and collaboration for design teams
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Sketch - Professional digital design for Mac.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Affinity Designer - Professional creative software, exclusively for Mac.