I am using a wind shield mount which i bought from eBay. Apparently I need 10 posts to be able to post links so i can't show you what it is lol. But it works specifically for the nexus one and its pretty good, does what it says on the tin.
I also have a car charger which I bought from Amazon. An HTC generic lol. Works though.
I also bought an FM transmitter ($10) in the store (impulse buy -_-) for my iPod Touch. Then when I realized the N1 had a 3.5mm jack, I started using it with my N1. The FM transmitter I have (random brand) works with AAA batteries (and also a charger). So when I'm in the car, I have it plugged in, mounted on the wind shield and plugged into the power with PANDORA for android running!
Yup, thats right, I have Internet radio from Pandora for my ride to work ^_^
I don't know about FM transmitters, but mine seems not bad for the price, though I hear there are better (though more expensive) ones like the iTrip.
The only other thing I really want now is a screen protector, maybe a case and an extended battery that adds more capacity but one where I don't have to change the cover. I had a 2600mAH for my G1 and that was simply amazing (solid use for 3 days without a charge!!).