I was recently asked to help clarify a design solution to team members and stakeholders. The project had been spinning wheels with everybody looking at the problem from different angles. What was needed was some good old 'users perspective' on how they would use the software, and this perspective needed to be seen by both the team and the high level stakeholder (read: does not have time to sit through paper prototype demo)
I looked at using keynote ( e.g. http://keynotopia.com/ ) but that would still require a lot of customisation for the specific project I was working on. I went back to Omnigraffle (this method is doable in visio) and came up with a super simple way to create animated mockups / presentations.
This is not an interactive mockup/prototype. The deliverable is more what I'd call and 'experience prototype' that can demonstrate conceptual usage, surprisingly quick and easy to produce, ideal for early stages in a project or the elevator pitch to clients.