PingtestPlugin
This plugin adds the command "/pingtest", which will reply a message to the player upon executing. It is simply a tool for measuring the lag between a player and server.
The time between when the player sent the command and when the player sees the reply is the amount of lag between the player and the server. The permission node for "/pingtest" is pingtestplugin.pt - it is available to all users if no permissions plugin is present.
This is my first CraftBukkit plugin, and I doubt it will need any updates or changes. I'm working on some more, so keep an eye out for them.
This plugin is copyright 2012 WinCamXP, and is available under the GNU GPLv3. Source is available on github.com/wincamxp.
This would be really useful, especially for servers that are in the U.S.
I love your painting skills. :D