I'd like to start by noting that I didn't come up with this. I discovered it on my own, but I've noticed a few others that also know this trick. This is mostly a tutorial for the beginners and anyone that doesn't know.
Aquifers are something of a contentious topic amongst dwarfers. Some hate them, some love them. I'm going to show you all how to make one into the heart of your fort's mechanical power by breaking a few laws of physics and thermodynamics. How we do this is by exploiting a very simple property of an aquifer tile; They generate AND absorb water. You can abuse this delightfully by digging out a pit that breaches the aquifer at several points and then use a screw pump to set water flowing between the aquifer tiles. Slap some waterwheels over the flowing water and Urist's your uncle; constant power. BE WARNED: once it starts, you can't stop it without involving !!Fun!! of some sort.
First, clear out a chamber that sits directly over an Aquifer. 'Dousing' with dug channels will do the trick if you're not quite sure where it is.
The single-square spaces on the left-hand wall are where you'll sit the gearboxes that hold the waterwheels
Once the chamber's dug out, channel down into the aquifer. I recommend doing it by rows from the north-most wall so you can build an entire row of wheels at once
I originally started going from left-to-right with all five rows due to the room's position, but then I realised it was far quicker to go north-to-south as I described above
Once the chamber's finished and filled with your rows of wheels, build a screwpump on the side that pumps from and to the water tank
The screwpump is the two barrel shapes on the north side
All that's left to do is start the pump manually. Don't hook it up to the waterwheels as all it will do is make a mess. If you can, order the pump to stop as soon as it's pumped once. That's all it takes to start it and it might wash the dwarf into the water tank otherwise.
That number there? That's all of the wheels generating power. Permanently. The value does not shift or stop, it just keeps going forever.
Hope this is of use to some of you, or at least provides a little entertainment.
edit:
Before anyone asks, the waterwheels are connected to a horizontal axle on the level above the one shown. You can do it on the same level, I just did it for tidiness.