That is called a TALK BOX. They are a bit tricky to use as they require you to set up a stand alone vocal mic along with your guitar signal. Once you have the hang of it though; it can sound very hip as evident by players such as Peter Frampton, Joe Walsh, Ritchie Sambora, and many others.

Check out this video of Stevie Wonder using the TALK BOX as well.

http://www.youtube.com/watch?v=VX5v-S_jGD4