The iConomy Plugin for the Hey0 Mod of Minecraft is very powerful. It allows users to hold a virtual currency. It helps to add a flavor to the server that that the vanilla game doesn't come with. Selling and buying ores that you dig up can come to be very useful to adding life to the game.
Edit* Forgot about the command /sign. I'll add a separate video on how to use it, since it's a bit more complicated than simple commands.