I'm looking for a word that is the counter part to "controller". So far, the closest I can think of is "controllee", but that means something else. Any ideas?
There's a lot of possible answers to this, depending on the context. If we're talking about people, puppet is a good term for someone being controlled by someone else. This is especially appropriate when the control isn't obvious, so the puppet master is controlling things "behind the scenes".
From a technical standpoint, I've also seen the word slave to refer to a machine or computer that is being controlled by another machine or computer. The commands are issued by the master computer, and carried out by the slave(s).