At home: Very little. MP3s, scanned photos, a few docs, MS Money data, Quake configs... I back them up whenever I feel like it, but not often. It's mostly for fun anyway. (Except the MS Money data - that gets backed up to floppy every time I use the app.)
At work: All data files. (Of course all my important stuff is on the servers, and they get backed up nightly.)
It's really not worth backing up the OS or applications, as these will have to be re-installed if the system crashes anyway. Besides, I install my OS and apps at home at least a couple times a year on my main PC.
Media: Home backups are on CD-R or sometimes to another HDD or PC via the LAN. Just depends on the situation. For keepers it's CD-R, but I don't do video or too many graphics so the 700MB per disk limit isn't hard to live with.
At work we use 20/40GB DAT or 30/60GB DLT on the servers. We also burn archive data to CD-R for offsite storage.