"Green Onions" (Booker T. & The MG's)

"Green Onions" (1962) is a soulful, deceptively simple groover and probably one of the most famous instrumentals of the 20th century.
It also was something of a throwaway tune.
Stax Records' house band (consisting of keyboardist/multi-instrumentalist Booker T. Jones, guitarist Steve Cropper, drummer Al Jackson, Jr., and bassist Lewie Steinberg) were scheduled to play on a session for rockabilly musician Billy Lee Riley.  But Riley never showed.  Because producer Jim Stewart already had the session time booked and tape ready to roll, he just recorded the musicians jamming on a sultry blues tune called "Behave Yourself."  Listening to the playback, Stewart liked what he heard and decided it should be released as a single.  
Only they needed a song for the flip side.
Putting their heads together, Cropper reminded Jones about a bluesy little piano riff he'd been noodling around with a few weeks before and suggested they might do something with that.
NPR's David Dye interviewed Cropper in 2005, who had this to say: "So (Jones) goes out on the organ and starts playing this riff.  Two cuts later, we had the 'Green Onions' that you know about today."
The single, which Stewart credited to Booker T. & The MG's (which stands for "Memphis Group"), famously got flipped by DJs to its B-side, and "Green Onions" became a massive hit—something the then 17-year-old Jones was only peripherally aware of.
"I started getting ready to go to Indiana University," Jones told NPR's Dye in 2009.  "I didn’t pay too much attention to (the record)—I liked the way it sounded.  But I was focused on being a freshman at Indiana at the time."