Invision - Prototyping and collaboration for design teams
Haiku - Haiku is an open source OS catered specifically to the needs of personal computing.
Zeplin - Collaboration app for UI designers & frontend developers
Creo - Creo is a modern design tool for Mac which combines design and development.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Flinto - The prototyping tool designers have been waiting for.