Speech Jammer is an app that will cause you to stutter, drag out your words, and literally jam your speech.
The way it works is simple: put on headphones, turn up the volume, hit the jam button, and start speaking. The app plays back your voice with a slight delay making it nearly impossible to speak continuously.
You can also record your slow speeches which are saved to an easily accessible library.
The novelty wears off, but it's still a great joke/prank to pull on unsuspecting friends.