The Speak for Yourself communication app is 50% off today only, in honor of Undiagnosed Children's Day. This app can start small enough to be used by babies, and grows large enough to be used by adults. On sale today for $99.99.
This link will take you to a page with a link to a blog post that explains why this app tops the other big name apps: The Best Communication/AAC app.
And this link will take you to their iTunes page: Speak for Yourself in iTunes.