[quote name='Scobie']Don't mess with the FM transmitters, guys... hardwire it into your head unit's aux port; well worth the effort. Also, you may want to consider a ground loop isolator if you're doing this.[/quote]
I couldn't agree with this more. I know you basically said it's not what you're looking for, and it's probably more money and effort than you thought you wanted to spend, but goddamn is it worth it. I did it in my car a few months ago, and it's the closet thing I've had to a life-changing experience-forget any important life lessons or anything, having your MP3 player properly set up in your car is one of the few things to make you a better person.
Seriously though, it's easily the best option, and if you don't want to do it, at least go with a cassete adapter. It might not sound great, but it's leagues above FM transmiters, which from my experience cause way too much hassle and most are pieces of junk.