Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
Sketch - Professional digital design for Mac.
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.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.