Wireframe.cc - Sweet and simple online wireframing tool.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
WireframeSketcher - WireframeSketcher is a wireframing tool for creating wireframes, mockups and prototypes for desktop, web and mobile apps. It's a desktop app for Mac, Windows and Linux, and a plug-in for any Eclipse IDE.
Sketch - Professional digital design for Mac.
ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
Invision - Prototyping and collaboration for design teams