Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Sketch - Professional digital design for Mac.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Affinity Designer - Professional creative software, exclusively for Mac.
Invision - Prototyping and collaboration for design teams