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.
Axure RP - The prototyping and specification software used by 87% of the Fortune 100. Beyond mockups, design interactive prototypes for business solutions, websites, and applications.
Sketch - Professional digital design for Mac.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Affinity Designer - Professional creative software, exclusively for Mac.