Bukkit

v0.6
+ New permission node to deny others from hiding or showing them (hideAll.ignore)
+ Custom messages can now include colours http://www.minecraftwiki.net/wiki/Formatting_codes

v0.5
+ Added a config file.
+ config options for custom messages when hide/show or cooldown.
+ config option to set the cool-down delay.
+ Permissions added for hide/show - hideall.hide & hideall.show
+ Config option for custom Item to replace the default clock.

v0.4
+ Cooldowns for hide and show.
+ Right click while holding a clock will show all online players.
+ Left click while holding a clock will hide all online players.

PVP must be enabled for the hide player when hit feature to work.