Pavegen manufactures tiles that use piezoelectricity (I think it's safe to assume, though the site doesn't say) to "harvest" electricity from motion. They have a variety of posts for tiles installed in offices, at rail stations, schools and colleges, etc.
Specifications are given nowhere on the site, so I've had to use information from secondary sources (though I should be able to take a look at real-time generation at an office during regular hours by going here).
Looking at a couple of videos it looks like a tile deflects maybe a quarter of an inch (call it 0.006 meters) under the weight of a normal size adult male, say 76 kilograms and a weight of 750 newtons (round numbers here, they're just rough estimates). So the work done by 750 newtons through 0.006 meters is 4.5 joules. I'll assume that the tiles are 67% efficient at turning this work into electricity, making 3 joules available per step.
The source linked in the previous paragraph says that, at the West Ham Station installation, each tile got about 5 steps per minute and that a tile will produce about 75 watt hours of electricity "on a good day." The information is confusing because it says that 50 steps per minute yields 6 watts which, in 24 hours, would yield 144 watt hours. But never mind. OK, back to the calculations. 3 joules times 5 steps per minute/60 second per minute is 0.25 watts. So, each hour would deliver 0.25 watt hours. Doing this for 24 hours would yield 6 watt hours. 50 steps per minute would yield 60 watt hours, close to the 75 cited above.
Anyway, the 6 watt hours would cost me 0.006 kilowatt hours * $0.16/kilowatt hour, or $0.00096 (96 thousandths of a penny). Though the price of a tile isn't given, it's pretty certain to take a very long time to pay off financially at a penny every 10 days or so.
I take something like 5000 steps per day, and would generate 5000*3=15,000 joules, or 3.6 kilocalories (that is, food type calories) were every step on a Pavegen tile. I step about 2.5 feet per step, so 5000 steps is 12,500 feet or about 2.4 miles. Here, we read that a 180 pound man burns about 100 kilocalories per mile as he walks, so I'd burn about 240 kilocalories (in other words, 240 kilocalories of food would be eaten) to walk this distance. I'm sure it's not exact, but here we read that, in order to feed me with 240 kilocalories, 2,400 kilocalories of fossil fuel is used.
So, putting it together, if the Pavegen system harvested the energy from every step of my walking, 2,400 kilocalories would go to produce 3.6 kilocalories of electrical energy. Of course, this isn't fair to Pavegen since I'd also, presumably, have gotten where I was going. On the other hand, I paid for the food to do that walking.
In the end, generating electricity through human effort is just not very efficient in comparison with other methods. If we're using muscle power for exercise and the effort would otherwise simply heat the environment, there may be a place for it (though I doubt it would ever pay off financially) but as a green power source, sorry, it's no sale.