Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Invision - Prototyping and collaboration for design teams
Sketch - Professional digital design for Mac.
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.
Affinity Designer - Professional creative software, exclusively for Mac.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.