Talk Assist is an alternative speech speech aid that provides a voice through text-to-speech software. It's created by Mubaloo and is free on the iTunes App store. I know that it's compatible on iPhone, iPod and iPad, but I'm not entirely certain of whether it's available for other devices.
Now, I'm usually somewhat functional verbally, so I'm not sure how this app works for daily interactions for non-verbal Autistics. I did run Talk Assist through scenarios where I lose verbal skills and need to use alternative methods of communication. I was able to type in and save several sentences and phrases for future use, making them easier to access in an emergency.
I found that the controls are simple and user friendly. I didn't quite like the voice, which is a rather mechanical male voice lacking punctuation. However, I think that it is a suitable app for at least temporary non-verbal Autistic adults.