This is a simple Bukkit plugin that builds on top of Bukkit's Vanish API (as of Craftbukkit 1.1 RB 4).
This plugin will allow privileged users to list vanished players, as well as see them as if they weren't vanished.
It also has a faux logout/login routine that makes it look like a vanished player left the server.
Minimal setup. The four permissions are as follows:
vanish.vanish | Allows a player to vanish
vanish.list | Allows a player to list vanished players
vanish.seeall | Allows a player to see vanished players
vanish.vanishother | Allows a player to set another player as vanished
All of the permissions are already assigned to OPs
To vanish, type /vanish. To see vanished players, type /vanish list. To make another player invisible, type /vanish <playername>.