Patternry
Collaborative design patterns.
Process On
Web-based diagram tool.
Webflow
Browser-based, drag-and-drop, responsive website tool.
FROONT
Browser-based, responsive website prototyping.
UX Pin
UX Design & Wireframing Tool.
Appcooker
http://www.appcooker.com/overview/mockup-wireframe-prototype-iphone-or-ipad-apps
iPad app for wireframing and prototyping iPad and iPhone apps.
Wireframe.cc
Simple, browser-based wireframing app.
Keynote
http://www.apple.com/uk/apps/iwork/keynote/index.html
Flexible presentation app with prototyping and wireframing uses.
HotGloo
Browser-based prototyping and wireframing app.
Balsamiq
Sketching tool for rapid wireframing.