I was trying to think of what we could use Ender Chests for on the server, given that there is only one Ender Chest for the entire server (not one per player, as would be ideal.)
Post office is a neat idea. Would also be cool to have one as, say, a portable Public Works. People would donate stuff into it, and you can use whatever you need--just be nice and try to put stuff back later.
Although it's likely the mod community will come up with some variations on this for servers. I'll have to keep an eye out!