Philips PET824 8.5-Inch Portable DVD / DivX Player
Worth every penny!
We do a lot of driving and my kids are generally very good in the car.  However, our recent trip was an 11 hour drive and that's hard for even the best of kids.  We used the car adapter which worked perfectly, haven't used the battery yet.  The screen quality is excellent and it was very easy for my kids to use.