Wireframe.cc - Sweet and simple online wireframing tool.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
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.
Invision - Prototyping and collaboration for design teams
Saola Animate - Easily create engaging HTML5 animations and interactive web content without coding skills
Zeplin - Collaboration app for UI designers & frontend developers