The guy on the next hill along from us, has just put a tiny hydro generator at the bottom of his drive. like ours its around 1/4 mile long and a good steady gradient, he has a 4" pipe running from the brook into the pipe and down the drive. It then goes into a turbo like thing and turns this small motor. To be honest its pretty neat and kicks out around 300W of power. Not bad for free water, but then we dont count the £400 he spent on pipe and fitting, £75 for the turbo thing, £650 for the armored cable back upto the house. £200 for the guy with digger to lay the pipe.
I went to a house a couple of years back, the address was 'Mill Farm Close' (so a bit of a give away).
It was a semi-detached house, and dead on the join of the two houses was the original water feed to the mill that used to be there.
So they got together and fitted a water turbine - it's only 2KW - but it produces 2KW continually, no matter the time of year.
They share the power between them, and if any is 'left over' it's dumped to storage heaters in the bed rooms.
The small amount of power isn't worth selling back to the grid, who don't pay very much.