I just had this idea that, on servers, it would be really nice if there were writable messages and mailboxes to leave them in.
In my head, the mailbox would be like a chest, locked to the person who made it, but others can deposit things (packages, letters, etc.)
I think this would be great, because a lot of times, you run across something that is really fantastic, and you would love to express your appreciation, make a comment, or leave something, but there isn't a way to do that unless the person who made it is online.
Additionally, letters could be a really good way to leave more information than you can with signs, etc., which don't have a lot of space.