This myth, particularly popular among psychics, is widespread and has been used in advertising campaigns and popular culture for years. And while only a small number of our brain's neurons are actively firing at any one moment, that doesn't mean the rest of our brain is just sitting around.
Brain imaging research shows that just about all of our brain is used over the course of a day. Brain function isn't extremely localized: different parts of the brain handle different functions, and this is spread throughout the brain -- there is no 90% of unused gray matter lying around up there.