I bought a 17" iMac recently, and it came with Apple Pro Speakers, which are nice, but I have some 20-watt powered studio monitors, so I don't need them. And they don't really work with anything other than the iMac, and Power Mac G4.
I happened to notice Griffin has the iFire, which changes that. Basically, the speaker cable for the speakers sends both power and signal. The iFire combines the two, using a standard 1/8" stereo audio cable for the audio signal, and FireWire for the power. You can plug the audio cable into any audio source, and the FireWire cable into either the FireWire jack of your Mac, or the iPod power adapter.
So now I have a compact stereo system to travel with. I bring the iPod with me anyway, so I just add the two relatively small speakers, the iPod adapter, the iFire, and a couple of cables.
The one drawback is that I don't have readily available iPod power. I either need to make sure the iPod is powered up enough, or plug the iPod into its adapter and the iFire into my laptop (since I always travel with that too).