According to Nick Craver, hamsters power StackOverflow and related sites. At any given time, how many run the site? What is the average turnover (how often do they need to be replaced)?
Each hamster consumes about 400 calories a day, which is just under 1/2 watt/day, or 12 watt hours. Assuming a 50% fuel to energy conversion (pretty optimistic, but let's assume these are elite hamsters that prefer the hamster wheel to running about the cage) then we can get about 6 watt hours out of an active hamster, on average, each day. The motor in the hamster cage is, sadly, only about 80% efficient, and the DC/DC converter to the power buss bars is a nice efficient 90%, but then we have to upconvert the DC buss once more before powering the data center, and that sucker only runs at 80% efficiency. So we only end up with 3.5WH/day out of each hamster.
Each server consumes about 600 watts per hour, and there are about 18 servers, so we're running about 11KW, which is 264KW hours per day.
So, excluding HVAC power, lights, infrastructure, and other overhead costs, Stack Exchange must be running at least 76,000 hamsters.
Hamsters have a MTBF of about 20 months which means that they have to replace about 125 hamsters per day - about one every twelve minutes. This system, unlike feeding and cage cleaning, is not automated, so they probably have two or more hamster technicians on 24/7 call to handle this and a few other relevant duties.
Of course this doesn't count hamsters on disability, medical, vacation, or maternity leave, which may require another 50-70 thousand hamsters. Further, I'm generalizing on power requirements - with peak usage energy storage, sane power schemes, and other techniques they could be easily using half this required power on average.
That may seem like a lot of Hamsters, but you have to realize that Stack Exchange is handling 95 million page hits per month - so each hamster is powering 41 page hits a day. Even though Google employs more pigeons, I suspect that Stack Exchange is more efficient on an animals-per-page-served basis.
I dare you to teach a hamster to serve 41 pages a year, nevermind each day.