If you're looking for AuthModClient, it can be found on BukkitDev or GitHub
The basis of this plugin may sound a little bit silly, but it is valid in some cases.
People try and succeed at stealing Minecraft passwords all the time--enough that a staff member on your server may be affected. Malicious people (i.e. script kiddies or "l33t h4x0rs") like to go to a server, try to bruteforce admin passwords, and use admin accounts to be destructive.
In this situation the plugin becomes useful.
AuthMod uses a certificate file that is stored in a user's .minecraft folder as a way to verify that any given player is really who they appear to be. There are two parts to AuthMod; the server plugin and the client mod.
The server plugin makes sure that people required to have AuthMod installed have it installed, and that the certificate is valid. The client mod sends over the encrypted key for decryption by the server.
If a player that is marked as an AuthMod user doesn't have it installed or has an invalid certificate, they will be kicked after five seconds. During these five seconds, they can't move, they can't chat, they can't do commands, they can't break blocks, and they can't interact (right-click) on anything.
The only permission node to set up is "auth.required". If that permission is set and a player with that permission node connects, they will be expected to have AuthMod installed with the correct certificate, else they'll be kicked. Because this uses the new Plugin Channels, there is no crashing of the client or the server if either doesn't have the Mod/Plugin.
The first time AuthMod is loaded, it will generate a private and public key, stored as private.key and public.key in the root directory of the server, respectively. After the key is generated, make a copy of public.key named authmod.key, and give this to your AuthMod users to put directly in their .minecraft folder.
If at any point the key is compromised, simply delete both private.key and public.key. New ones will be generated the next time the plugin loads.
Source code and a compiled binary can be found on GitHub
Networked Hunger Games plugin that has many of the same features as the big servers (and more!)
Networked Hunger Games plugin that has many of the same features as the big servers (and more!)