This plugin lets players claim chunks.

Players can share chunks with friends so that they can build on the same chunk together.
Likewise, players can surrender their claim on a given chunk.

The operator can set parameters in the properties file to limit the number of chunks a player can claim.
The operator can also enforce a "borderlands" zone between adjacent players' chunks so that other players can move freely through the world.

In-game commands are as follows:

{place a block} - If the chunk is unclaimed, this will cause the player who placed the block to claim that chunk. Placing items like paintings and armor stands doesn't count; the player must actually build on the chunk to claim it.
/CHUNK HELP - Will display a help message outlining the following commands to the player who issued this command.
/CHUNK RELEASE - The player who issued this command will release his claim on the chunk he is standing on.
/CHUNK SHARE [PLAYER] - The player who issued this command will share his claim with the chunk he is standing on with the named player. Note that this cannot be undone by the original player; once a chunk is shared among players, a player can only release his own claim on a given chunk.
/CHUNK SHOW - The player will see a border around of all chunks within view that he has claimed.
/CHUNK OP HELP - Will display a help message outlining the following commands to the player who issued this command.
/CHUNK OP WHO - Will display the names of all players who have claimed the chunk you are standing on.
/CHUNK OP RELEASE [PLAYER] - Will release the claim to the chunk you are standing on by the named player.

IMPORTANT: This plugin does not protect chests and armor stands. Use an anti-theft plugin for that. Or tell your players to build a secure base.

To install this plugin, copy the plugin JAR file to your server's plugins folder.