For my car I'm debating the Mobile Cassette Adapter or the TuneCast II Mobile FM Transmitter. I'm kind of leaning towards the latter, since I think I can use it with my portable stereo in the office too. It doesn't look like it has audio in jacks to cable the iPod in directly, just a Mic jack. And I'm not sure how that'd sound. It does have cassette, but the doors close fairly tightly. Any reason I *shouldn't* go for the TuneCast II?
For power there is the Auto Kit for iPod w/ Dock Connector or Mobile Power Cord for iPod w/ Dock Connector. They look very similar, except one has a built in amp I guess. I don't know what the point is if I use the FM transmitter. I'll probably get the Speaker and Headphone Splitter, just because.
As for add-on battery packs - I don't know that I see the need, that I'd be away from some power source and needing playback for long enough. The TunePower Rechargeable Battery Pack looks keen and all, but even with a 50% discount, I don't know. What do current owners think?