The Keynote app is designed to make "world-class" presentations, complete with animated charts and transitions. I have found that this app is also a great way to share presentations with other professionals, parents, and outside agencies with parental permission. Currently, a speech therapist shared 4 presentations with me of a student's signs, as their sign is not the typical ones. Each slide can also be a video of the student demonstrating the sign.
Here are a few examples of how signs can be used with this app. (Note that this is not my student!)
This app currently sells for $9.99 from iTunes.