It's a myth, rather like saying you only use 10% of your muscles. You use various areas of the brain for different things, and there's a limit to how successfully the human brain can multitask, so the load is spread out over the brain over time.
Did you know that it IS true that you are carrying most of your visual load on one eye or the other, though? One eye does the bulk of the work while the other "coasts", and in a normally sighted person the eyes switch the load back and forth all day long.
I have to agree that the 10% brain usage is a myth. While some people's brains are far more developed than others, it does not mean that they use less of their brain. I think that Litteacher nailed it: we simply do not use our entire brain at the same time. Parts of our brains are simply meant for specific functions. If we are not activating that portion of our brain, it has no reason to be "working" at the time.
Apparently the idea that people use only ten percent of their brains is a myth, and, if you think about the claim, it doesn't make a great deal of sense. The brain has evolved over an enormous length of time, and it makes little sense that non-functional, non-useful parts of the brain would have survived the process of selection. To say that I use my fingers most when I type is not to say that I don't use my arms and my brain; each part of the organism is important to all the other parts. Yet apparently the brain can in some cases adapt to the loss of function in some parts, although the degree of such adaptability would seem to be very limited.
Here's another useful link: http://www.snopes.com/science/stats/10percent.asp
If you search on Google for "ten percent brain myth," you will find many other discussions of this matter.
No. Everyone uses 100% of his or her brain.
Isaac Asimov (1920-1992) once wrote an article regarding this (which I cannot find , unfortunately) which reemphasized that different areas of the brain do different things; in addition to the higher cortical functions of reasoning, the brain, as the primary organ for the body's nervous system, is busy regulating heartbeat, breathing, motor functions, and all the other critical neurological functions for survival. Additionally, the brain is busy processing auditory and especially visual data. I believe he understood the 10% statement to mean that only 10% was used in the higher cortical functions -- and that the remaining 90% was busy on autopilot keeping us working. If we could use 100% of the brain as cortex, yes, we'd be superintelligent! ...but also 100% dead from lack of oxygen, heartbeat, nervous and sensory processing.
The link suggests some origins for the "10% Brain Myth:"
How is any of this scientifically provable? It can't be determined that 10% of cortical function is used. Scientists make assumptions based on MRIs and like #4 said it's because we don't use all parts of our brains simultaneously.