I have a Griffin iTrip, as above. Quite impressed with it, but I found I never really used it... almost new condition, yours for £10 including postage, if you'd like to try one out on the cheap (shout if you'd like a couple of photos).
They are clever little things, with good sound quality, but I found it a little irritating on long distance trips. As you travel you move into areas which have stations broadcasting close to what was a clear frequency earlier in the trip, and you have to re-tune. Might not be an issue for you, depending on how you intend to use.
I now prefer a simple 3.5mm jack cable from ipod into aux input on car stereo.