ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Wireframe.cc - Sweet and simple online wireframing tool.
Invision - Prototyping and collaboration for design teams
mockingbird - Mockingbird helps you you create and share clickable wireframes. Use it to make mockups of your website or application in minutes.
Zeplin - Collaboration app for UI designers & frontend developers