I'm working on an iOS 7 project and was in need of a quick prototype of an interaction I was trying to get down on paper.
Out of the various tools and methods for prototyping interactions and animations, Keynote seems to work best for me. I've just really started exploring and am just really discovering the power of the Build inspector. It really gives you a lot of flexibility in terms of timing, a few pre-defined animation curves and other parameters. As I mentioned, I literally just through this together, so there is tons that could be improved.
I've included the (awfully simple) Keynote document for anyone to check out if they're interested