I'm a fan of CDs. They're easy to back up, have nice little cases, and are much harder to lose because of it. Write-protection is hugely important, too.
Many years ago, I attempted to destroy one of those ubiquitous free AOL CDs that showed up in the mail. Scratched it up beyond recognition, put two holes in it with a hole punch, and it still worked - even in an ancient, caddy-style CD drive.
