Can you put music you already have on them? Or do you have to buy all the music from iTunes?

No, you can use any music you have. It does not have to be from iTunes, otherwise iPod sales would be completely dead.

But you have to use iTunes to load the songs onto the iPod. Not a big deal.

Are there any players out there as good or better for less money?

I really like the creative mp3 players. If the ipod I am getting wasn't free, I would get one of them.

edit....I strong dislike ipods, but it could just be how I like to revolt against fads

Creative has a bunch of good ones, Sandisk, Samsung, etc.

They are pretty hyped IMO. I have one, and don't get me wrong I love it, but there are now mp3 players with more memory, similar features for less cash. The only thing that other players lack in my eyes is a really effective "control system" (such as the click wheel on iPods) and the cool factor of the new Nano.

Alright...*dons flame retardant suit*

I have a creative zen micro. I personally love it. Any questions, ask, but I have no complaints.

I'll assume the best way to play one in a car with no audio input is an FM modulator. Are there any that work really well as far as sound quality goes?

all FM modulators are pretty much the same. Some claim that the ones that plug inline are better but it wont be much better, IMO.

With a little bit of electronics knowledge you could do what acid did and make a input on your stock unit.

I'm with logan, that creative zen micro is SWEET.. my friend has one. One of my other friends has an older creative. I like both of them a LOT.