Simple LAN chatrooms.
On a cruise ship with exorbitant wireless access fees, when all you need is a way to stay in touch with your family or friends? On a plane, and don't want to pay the insane Internet rates to stay in touch with others on different parts of the plane? Simply launch LocalChattr, click "Start Server", and you're set! You get access to a fully-featured chatroom hosted on the your own computer, meaning anyone on the same network can join and send messages without paying a dime, since you don't actually need Internet connectivity.
Updated on Jun 27 2014