Area 450 has several guides to adding hardware to the Sampo DVE631CF DVD player. Even if you don't own this model, the firmware for it has been ported to many, many other models (with annoyances like macrovision and region locking removed). This player had built in support for an IDE device (a flash card reader) so a standard IDE drive can be slaved to the dvd drive and the player can read from a FAT32 formatted disk. The player decodes mp3s and VCD files, so it's possible to turn it into a cheap mp3 jukebox, or store movies in vcd format. I hope that when DiVX support becomes more common in DVD players one of them will be able to support a hack like this. It would be really cool to have 100+ movies built in to my dvd player."
THE AREA 450 SITE
I don't have this DVD player but it shouldn't be too difficult to find.
I wouldn't mind trying this out.