The sound around you: things happening, machines functioning, people talking and moving. John Cage's 4:33 was an exercise in getting people to listen to ambient sound.
Also a cool thing to listen to, if you're interested in noise. I made an experiment while travelling from North to South London and recorded the ambient sound of the journey on a Nagra, and it's an incredible selection of bass noises, mechanical hums, screeching metallic noises and varying beats.