Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
Fabricating Alternatives - AI-powered card deck to help ideation & brainstorm
Balsamiq Cloud - Fast, approachable, collaborative wireframing
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Justinmind - Define awesome experiences through wireframes & prototypes.
VectorScribe - The missing toolbox for Adobe Illustrator.