The Know Your Meme Blog has a great introduction to the history and technology behind Auto-Tune, the audio effect first introduced in 1997 and made popular when Cher's single "Believe" made heavy use of the effect in a way that was never intended -- emphasizing the warbling of pitch correction rather than simply using it to sweeten slightly sour notes. A bit from Know Your Meme's article:
With the expense of the Antares Auto-Tune plugin and the relative lack of social networking sites at the time, Auto-Tune remained something of a trade secret among many record producers who simply wanted to save time in the studio. But as social communities, user-generated content, video tutorials, and open-source software development proliferated online in recent years, so did the availabilty, usage, and experimentation of Auto-Tune. In 2005, rapper T-Pain made his debut with the album Rappa Ternt Sanga. The album used Auto-Tune heavily, on every song; giving T-Pain his signature sound. His first single Sprung can be viewed here, although embedding has been disabled.
This fun video (featuring "Professor" Weird Al Yankovic) has further explanation:
Read the whole article here (includes a bunch of embedded videos showing the historical moments along Auto-Tune's journey). See also: the latest "Auto-Tune The News" episode, featuring Kanye and Ballon Boy.
Previous coverage of Auto-Tune on mental_floss: The World of Auto-Tune, "Cosmos" Documentary, Auto-Tuned Vocals by Carl Sagan, ft. Stephen Hawking (Seriously), and Winston Churchill Getting Funky.