We humans have managed to speed up the Earth's rotation enough to shorten each day by 10 microseconds
by impounding water behind dams in rich countries, which tend be at high latitudes.
The dams pull water away from the the equator, where it was spinning with a high linear velocity. By conservation of angular momentum the Earth is forced to spin a little bit faster when the mass of water is pulled inwards, just as ice skaters spin faster when they pull their arms in.
I’m cherry-picking here from Philip Greenspun’s recent entry (via mpt), but read the whole thing. I must say that I’m surprised that he can think that you can fail to notice the impact that humans have on the environment if you live in North America - I’ve seen New York - but I can see where he’s coming from.