Recs.
Updated
Invision is a prototyping platform built for designers. It allows designers to quickly create animated and interactive mockups.
Specs
Pros
Pro Design files are automatically synced and their version history tracked
Invision uses Invision Sync or Dropbox to automatically save design files on the cloud. Furthermore, it has version control built in. This is especially helpful if a team is going through different versions for a specific design and they want to return again and again to previous designs.
Pro Free plan
Invision offers a free plan to anyone that is limited to one project. While the free plan is limited to only one prototype and lacks any official support, it does allow the user to trial the software for as long as they like to decide on its merits and whether it is worth paying for the more robust options.
Pro Great built-in feedback features
Invision allows clients, stakeholders or any team members to leave comments and feedback inside the design which will be displayed as a point on the page itself. These paints can be discussed or even be turned into tasks. This makes it easy for designers to go through what they need to work on, and once it's done, the task can be checked off.
Pro Focused specifically on prototyping, speeding up and streamlining the process
Invision isn't concerned with exporting or deploying what you've created. Instead, it focuses on the process of refining your design. This leaves the developers free to add more features that a designer would find useful, rather than cramming in tools that a programmer would rather use.
Cons
Con Limited customer support
If you have questions you'll likely end up either working it out on your own, or having to rely on a third party forum. The Invision company gives precedence to higher paying users, so if you aren't on an enterprise or higher tier plan, your support tickets will be pushed to the back. The Invision official forums can be found here.