InVision describes itself as the world’s leading prototyping, collaboration and workflow platform. Given that it claims to be used over 1 million designers then this easy to believe. InVision allows you to easily create interactive prototypes by adding hotspots to uploaded designs and provides lots of great collaboration and presentation features, including annotations, tracked feedback and project updates. It has lots of cool features like automatic Dropbox syncing and user testing capture (which of course don’t come for free) and is probably the slickest prototyping tool currently out there. A free account allows you to set-up just the 1 project.