Invision - Prototyping and collaboration for design teams
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
Pixelmator - Pixelmator is an image-editing application for Mac and iPad.
Zeplin - Collaboration app for UI designers & frontend developers
Pixlr - Pixlr® makes it easy to transform everyday images into stunning works of art.