Malcolm dreams one night that he can levitate and wakes up to find that he is still able to. He teaches others to do it and it becomes a world-wide craze. Soon nearly everyone can do it and Malcolm finds he is no longer the centre of attention. Can he or anyone else find a use for this levitation other than for fun?
Almost any human can learn to levitate to about 1 cm above the ground. The actual height is given by H = WP/M where H = height, M = mass and WP = willpower. Willpower can be measured by psychological tests.
Lateral movement is possible and almost frictionless but requires some form of propulsion - it obeys Newton's laws. On a slope you will not accelerate downhill but instead move at a steady speed according to the angle. If the angle is steep you may still be touching the ground. If it's too steep, watch out!
Skating doesn't work because of the lack of friction in all directions. (Edit: As has been pointed out, you could de-levitate, push off then levitate-and-glide. What I meant was that you can't push off with your feet while levitating)
Levitation is quite exhausting. The world record is 20 minutes and 2.3 cm.
If you fall from a height, your momentum will overcome your ability to hover so your injuries will be almost the same.
Is there any conceivable application for this ability? (EDIT - Specifically in a way that would most benefit local populations or humanity in general. This could be measured in economic terms or in terms of, say, how the the invention of the wheel changed everything about civilisation.)
I exclude scientists developing an improvement in the future. I want to know if there is an application right now under the above conditions.
Answers to comments (ongoing, I will treat them one-by-one in no particular order. Remind me if I missed yours)
MonkeyZeus - If you are wearing shoes with thick soles then you will not rise unless you manage to levitate right out of your shoes. This is because the insoles of your shoes count as the ground while you are wearing them. The world record was achieved with bare feet and virtually naked.
Chronocidal - If you try to levitate whilst lying down, your head will rise until you are upright and then you will levitate the normal distance. Note: I've had to change this for consistency - I don't think it invalidated anyone's answer.
Dewi Morgan - An equal and opposite force on the ground does occur. It's just as though you are standing on it with your feet although more spread out as if you were standing on a large soft rubber ball.
Nuclear Wang - You do not accelerate down a (gentle) slope but you do move down it at a constant velocity (Think Lenz's Law). I have to check but I think the speed is proportional to the sine of the angle. When the slope gets beyond a critical point, you cannot maintain a small enough vertical distance between you and the surface. At this point a catastrophic failure occurs and you begin to accelerate.
Kamil Drakari - The only friction in a horizontal direction is from the medium you are levitating in. You cannot use will-power to move laterally. Otherwise it's just like standing on a very slippery surface except that you automatically remain upright.
Dewi Morgan - In a train, when the train starts to move you will tend to remain stationary so hold onto something. On a plane's wing you would simply be blown off unless you are fixed to it.
You can levitate over a liquid but it will be pushed downwards and away. You can 'float' over dense liquids (such as lava) once you have displaced your own weight.
M.Herzkamp - Yes it is definitely more exhausting to levitate a higher mass to a given height. You can compensate for a large mass by levitating a smaller distance.