Players can create and use chairs on the Minecraft Survival Server, we make it possible to use stair blocks and signs to create chairs so that you are able to sit down on them. See the above image on how to construct simple chairs. In game it will appear you see yourself as sitting a bit little higher than you are supposed to, however other players will see you sitting normally though. Chairs have to be created using stair blocks with signs on each side, you can create chairs up to three stair blocks wide for sitting on. If you wish to sit down on a chair, you simply have to right click on the stair block. If the stair block does not have a sign on each side, you will not be able to sit down. To stand up from sitting in a chair, you have to press your sneak key, you can also destroy the chair to stand up. Commands available are: /chairs on - Allow players to enable chairs for themselves. /chairs off - Allow players to disable chairs for themselves.
It's just called "Chairs" google "chairs plugin minecraft". One of the top links. It's on the Bukkit dev website.