Yes, this is what the iSub was. However, it doesn't work with Tiger 🙁 Apple apparently are working on it, but the last update bought no changes.
Other than that, there are none that I am aware of, and there's no way of using the headphones connection at the same time as the built in speakers to drive a subwoofer as far as I'm aware.
TBH, the built in speakers are great, but you'd be far better off just getting a discreet set of computer speakers...