Land Manager allows you to easily setup cuboid areas in the world which are protected against players placing and breaking blocks.
These areas can then be allocated to players by an OP or claimed by the players themselves (if configured).
Only OPs or area owners (if configured to for certain commands) can modify areas as well as players in Creative if the relevant config is enabled.
There is 1 item that this mod adds, and that is the Admin Tool. This item simply allows you to select two locations to define a cuboid area. Once you select the second location, you are prompted with a GUI to set the name of the area and whether you want the area to extend to the top and bottom of the world.
You can find all of the commands on the wiki here.
Thanks to 8BrickDMG for the logo and assisting with the mod idea!