The McGurk effect is mind-blowing. It involves showing a person's lips making the shape of one sound—like "bah"—while the audio is actually the person saying "fah." What's interesting is that your brain changes what you "hear" based on what you see. It's "bah" all the way through, but when we see "bah" our minds transform "bah" into "fah."
The effect is named for researcher Harry McGurk, who published a 1976 paper with John MacDonald entitled "Hearing lips and seeing voices." McGurk and MacDonald described how speech perception isn't just about sound—it's also affected by vision, and the integration of the two.
What's most interesting about the McGurk effect is that, even when the viewer knows what's happening, it still works. In other words, even thought I know it's an illusion, my brain can't seem to turn off the effect. Note: Some people are not susceptible to the effect; please leave a comment either way!
Here's a nice BBC segment illustrating the effect (jump to 0:30 if you just want to see the effect in action):