Rather simple and crude, but can be effective!

Upload a picture of a frontal view of a face, mark the key points of the mouth, and attach audio by uploading, recording, or using your mobile phone

It creates a simple animation of the mount moving to the sound of the voice, somewhat like what Conan O'Brien used to do on his TV show. It is a very humorous effect, but is often used to provide users voices to almost any photo (e.g. images of the Lincoln Memorial, or the demo on the site of the talking alpaca).

A newer feature (free at the time of writing) is the ability to export a blabberize as a video (exports as MPEG). Also, you can now record multiple characters, so you could generate a dialogue between different characters.