We are all familiar with "MY BROTHER USED MY ACCOUNT DERP". So I remembered this plugin from a server I was on a while back.
http://dev.bukkit.org/bukkit-plugins/loginsecurity/
This might be good for players who know that their account could be used by another person (and so those whom it happens to again have not excuse).
An extra layer of security never hurt anyone. And it can be configured to be completely optional (which I would prefer).
