Mozart effect

The Mozart effect is an infamous bit of pop psychology that posits that listening to the music of Wolfgang Amadeus Mozart will help your plants grow or your chickens lay more eggs or increase your intelligence. It was first proposed by Gordon Shaw, Frances Rauscher, and Katherine Ky in 1993 and has been turned into a veritable cash cow since then. It is a great case study in how a pseudoscientific "factoid" becomes embedded in the public consciousness despite its flimsy evidentiary backing.

Failed replications
The first paper proposing a "Mozart effect" was Rauscher, Shaw, and Ky (1993), which found an increase in scores of about 8-9 points on spatial tasks taken from a Stanford-Binet IQ test for subjects who listened to Mozart's Sonata for Two Pianos in D Major. A number of replications were attempted, with some finding significant but smaller improvements than the original study and others finding no improvement. A 1999 meta-analysis found no improvement for general IQ tasks and a minor, but statistically insignificant, improvement within the variability of a person's IQ task performance for spatial tasks. Further research suggests that the minor effects may be due simply to the improvement of mood after listening to the music rather than anything special about Mozart specifically.

Popular incarnation
While the original claim for the Mozart effect was only in regard to a temporary (approximately 10-15 minutes after listening to the music) increase in spatial skills, this soon transformed into the pop psychology formulation of "Mozart makes you smarter!", as well as the inverse "Rock music makes you dumber!" It became a fad among marketers hawking baby products to sell headphones and CDs to pregnant women with claims of producing child geniuses by playing Mozart to them in the womb. In 1998, then-governor of Georgia Zell Miller proposed a bill that would provide every mother in the state with a Mozart CD or tape. In 2007, a team of German psychologists and neuroscientists shot down the Mozart effect once again after being flooded with numerous requests to check its veracity.

What should you listen to instead?
Listen to Bach instead. He's a much better musician anyway.