PrisonCraft

List of features:
-added actual jail groups
--added a tool to link cells with a jail name
-added a jail view block
shows all players jailed in a jail block
-added the ability to give a reason for jailing a player
-teleport players back if they are more than 30 blocks from where they teleported into the jail
-reorganized/condensed some of the commands

Caveats

Pointless and "dangerous" on singleplayer worlds, as you can only jail yourself and then couldn't unjail yourself.