Joe IP Tracker
This is a plugin for Minecraft Bukkit servers. Tested and still works in 1.7.2.
What does it do?
This plugin reports related player accounts as they login (to players with permission). It also has a cool feature of reporting the geographical location they are from (if that information is available at the time).
Related accounts are accounts that login from the same location (could be friends, family, or sharing of accounts). This is useful for admins to know. For example if one account has admin or creative privileges, they might be tempted to share among themselves.
How can I configure it?
Give players (or admin groups) "joe.iptracker.notify" permission to receive notifications in game. There is an in-game "/ipt" command also you can use to check version or lookup a player specifically. To grant access to that command give permission "joe.iptracker" also.