iPods have a docking station base than offers a "line-out" jack. You can then plug this directly into a regular RCA amplifier input.
Alternatively, there are endless slot-in units that accept iPods and do that.
One of the things I really like for both portability and quality at a reasonable price level is the Cambridge Soundworks Model Twelve. This has two bookshelf speakers and a subwoofer (which is also the carry case) and a small integrated amplifier. It connects up much like a large component system, and yet can be taken on the road etc...
I have one installed on my boat and drive it from my iPod by using a dock and cable into the RCA port. I also use another on my computer for iTunes, for my SW/FM radio and for listening to NPR over the internet. Awesome and all for $399.
One of the other classic approaches right now is to use a plug in FM transmitter that uses the earphone socket and transmits on FM to your tuner/radio. I've tried these, but found the quality to be very poor and living in an area that has congested FM frequencies it is almost impossible to get good reception.