No mention of the "Detroit Locker", IMHO one of the more clever automatic-locking differential designs --- it uses a pair of dog clutches which are normally engaged, one for each side, and the wheel that spins faster is allowed to. It's self-contained and requires no additional controls, just like a regular differential. I think the only reasons why they didn't become more popular are due to their cost and the noise they make --- a very noticeable clunking on tight turns as the outer wheel tries to go faster and unlocks, then locks again as the vehicle straightens out.
Detroit locker is effective, but I wouldn't really call it clever. For that I think of Torsen, the Torque Sensing differential that uses planetary gears. The problem with the Detroit is primarily noise, as you mention. The problem with Torsen is that it doesn't work in the presence of near 0 traction on one of the outputs, so people will drag the hand brake in that case.
http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differen...
Having driven a "4WD" with all three differentials of this type, I now wonder if it would be classified as 4WD or AWD.