antipop is a LaunchAgent designed to prevent Mac OS X from turning off the sound system. On many laptops (particularly on MacBooks and MacBook Pros), the sound system emits an audible pop or click when it goes to sleep. This is particularly noticeable when a low-impedance or externally-amplified device is plugged in to the laptop's line-out jack -- users with sensitive headphones or amplified speakers often hear this pop when Mac OS X puts the laptop's sound hardware to sleep after 15-30 seconds of inactivity. This software stops this pop from occurring.
NB: in 2009 I finally upgraded the last of my Mac OS X machines to newer hardware running Linux. As a result, I no longer maintain any Mac OS X software, this included. Go use antipopd instead. Matthew Robinson was not as lazy as I was, and his solution is better. :)