AuthMe prevents people, which aren't logged in, from doing stuff like placing blocks, moving, typing commands or seeing the inventory of the current player.
The possibility to set up name spoof protection kicks players with uncommon long or short player names before they could actually join. Login Sessions make it possible that you don't have to login within a given time period.
Official URL : http://dev.bukkit.org/server-mods/authme/
Servers using AuthMe, page 7Servers: 166