ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
Invision - Prototyping and collaboration for design teams
PowerMockup - Mockup & Wireframe toolkit for PowerPoint
Zeplin - Collaboration app for UI designers & frontend developers
Wireframe.cc - Sweet and simple online wireframing tool.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.