Paul writes about hospitality industry token mapping.
Another example courtesy of the U.S. Postal Service: In my condo building, we have rows of tiny vertical mailboxes, which you open with your key. If they need to deliver a particularly large wad of mail for you, they put it in one of three large general-purpose mailboxes in the mailroom, and put the key for that large box into your tiny mailbox. This works similarly to Conor‘s suggestion about keying the wall safe to the credit card of the traveler who is checking into the hotel after hours.
However, sometimes I fear our postal worker is unclear on the concept. Last night in my mailbox I found a hastily scrawled note on one of those “we tried to deliver a package to you and failed” slips of paper, saying “Your mail is in box #2”. The key to box #2 was hanging in the lock, accessible to anyone who walked by. Of course, this still has more security than you get with your average unlocked outside-the-house mailbox on a post, since you need a key even to get into the building, but do I want my condo neighbors to be poking around in my personal copy of the Victoria’s Secret catalog? Ick.