Proof of Location consensus wiki
Which has inspired amounts of researches and has reached the state of early adoption.
XYO: "Location… is very, very important, because location or proximity is one of the few heuristics we found that's actually co-optable by two parties." (founder's interview)
Platin (discontinued)
While yet to realize to date, it has attracted curiosities.
Pretty Good Proof of Location, as described in Spoofs, Proofs & Jamming by Logan Scott (presentation), where the system in proposal would provide the proof-of-location of an object's whereabouts 5 minutes ago or so.
Blockchain Proof of Location by Bob McElrath, wherein describes a potential decentralized implementation.