Like many of the posters here, I've kept around good hardware that works because it works and it's already paid for (please ignore my credit card balances for now...)
My primary archiving box and storage server is a Mirror Drive Door Power Mac G4 tower, which is awesome because it holds 2 DVD drives and 4 hard disks, which is better than most other Apple towers (with the exception of the Mac Pros.) It serves up what I need with OS X 10.5 and whenever I end up needing more storage, I'll throw a SATA card in there to use newer, faster, larger drives.
Sure it's unsupported hardware, but it's solid, it's relatively compact (compared with G5 towers and Mac Pros) and doesn't gobble that much power (survives w/ a ~ 300W power supply.) It gets the job done, and gets no complaints from me or the wife about its performance. Yay for old hardware that works!