Note: Pupster no longer works as of WoW 2.0.
Pupster ("Puppet Master") is an addon for remotely controlling another player via whispers. You need two World of Warcraft accounts and a Logitech G15 keyboard on the client, or puppet, end.
From the master's keyboard you send specially formatted whispers to the puppet. You can run any slash command, lua code, or action on the action bar. Ahead of time, you set up a G15 keyboard to continuously tap the keybinding to process the Pupster queue. Pupster queues up the requests from the master and is smart enough to delay firing actions until they are ready.
In practice, Pupster works great. I tried it with a Warrior (master) and a Priest (puppet) and cut through the mobs like butter. It's fun telling the Priest to shield me, Renew me, and spam Wrath on my target all in a couple keystrokes, and then to focus back on doing dps and keeping aggro with my Warrior.
I've spoken to a Blizzard employee about Pupster, and unfortunately its legality is very questionable. Sure, there are a few statements from various "blue" Blizzard reps about how you won't get banned for using any addon that solely uses the in-game Lua API, and how G15 keyboards are acceptable so long as they don't lead to unattended play, however he told me all of these are moot if a GM feels something has crossed the line.
I do not use Pupster anymore, having gotten the multibox itch out of my system. I will however provide the source (for educational purposes only, and at your own risk of course) if emailed.