For myself personally I just keep the really important files on several computers and I'm looking at running an rsync mirror on a machine at work too.

If I was keeping someone else's (or company) data then I'd probably hedge my bets and go with nightlies on removable HD and weeklies or monthlies on DVD-R using a decent brand of disc burnt at a slow speed.

That said, there are probably as many different methods as there are likely to be responses to this thread!