I use my old PowerBooks for simple webradio & iTunes audio playback. I also use my PowerBook G4 17" 1,67 Ghz for my canon flatbed scanner since canon stopped to update the driver after Tiger...
I will probably use my two old PB G4's once I decide to convert my library of CD's (in ALAC-form) to the more portable AAC. It takes a while but keeps my main mac free and ready to use for other stuff.
(but otherwise it almost starts getting to be a problem, my macs just keeps spinning and my collection of old, used & a bit slow but still mainly functioning macs just keeps growing... Got 6 macs right now.)