A reader sent me this photo of a shared lock. It’s at the gate of a large ranch outside of Victoria, Texas. Multiple padlocks secure the device, but when a single padlock is removed, the center pin can be fully lifted and the gate can be opened. The point is to allow multiple entities (oil and gas, hunting parties, ranch supervisors, etc.) access without the issues of key distribution that would arise if it were just a single lock. On the other hand, the gate is only as secure as the weakest padlock.
EDITED TO ADD (9/14): A less elegant way to do the same thing.
A slightly different implementation of same idea: removal of any one lock allows locking bar to retract from pole and gate to open. And an interesting comment from someone who deals with this in his work.