These are not exactly a necessity but they are quite cool! http://dev.bukkit.org/server-mods/portecoulissante/ http://dev.bukkit.org/server-mods/retractablebridge/ http://dev.bukkit.org/server-mods/chairs/
The first two, we already have in terms of Craftbook, with gates and bridges, but they're just instant, and the third one is well... it does nothing