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