Adam linked us to this good story on Gizmodo examining the origins of the common misconception that we only use 10% of our brains. Neuroscience and psychologists researchers in the early 20th century attempted to quantify how much of our brains (by mass) that we use for everyday activities, to find a correlation between brain mass and cognitive capacity. That line of thinking endures, as a means to explain latent cognitive potential. Of course, we actually use virtually all of our brain, and recent studies have shown that most of our brains are active over the course of a day, even if not all at once. Further reading on the topic here.