I have a P3 laptop that sees regular use around the house as a portable DVD player/video player.
IBM Thinkpad 600x It's a PIII-500, upgraded to 384MB RAM and a 40GB disk. I found the optional DVD drive online and bought one.
It is currently running Mythbuntu 8.04. I found an X.org configuration file on teh interwebz to get the accelerated driver for the Neomagic 256zx to work properly instead of having it using the crappy default SVGA driver. It decodes DVDs without any problems. I can also play most video files, although not the latest H.264 encoded ones.
I also have an old 802.11b wifi card in it so it can get to the wireless router for filesharing and updates.
My next project is to get NES, SNES, and Genesis emulators up and running.
No, it can't compete with any computer made this century, but it destroys any portable DVD player out there for a fraction of the price.