I have become pretty impressed with the backup built into Windows 7 and use that to do backups of critical data to a network shared drive. It's pretty easy to use and can be scheduled which is brilliant for me because I'm generally lazy when it comes to backups. It's also free.
Some data also gets backed up to one of two portable USB drives, though that's not all that common for the above laziness reasons.
Before that (and on my XP systems) I'll use SyncBackSE from 2BrightSparks.
http://www.2brightsparks.com/syncbac...-features.html
It also does scheduling and network backups. It also has a few additional options in regards to filters and how the backup gets done as compared to the Windows 7 built in backup. The price is pretty reasonable at $32 too.
There's also a free version available from
http://www.2brightsparks.com/downloads.html#freeware which will do quite a bit but is missing some features like being able to backup open files. It's fine for doing simple data backups though.