Since most documents and source file or also stored in a VCS like SubVersion or sit on my ISP's server or the iDisk, like probably most of us, I'm not too meticulously when it comes to backing up my stuff.
Still backups are required once in a while so I did one last weekend.

Maintain several iTunes Libraries on one Mac

I like to keep PodCast and my music separated in different iTunes Libraries. Libra a Shareware tool Steve Roy has created, http://homepage.mac.com/sroy/libra allows conveniently to switch between iTunes libraries.

iTunes Library Backup

I haven't found the perfect way to backup my iTunes libraries but here is how I usually do this:
In iTunes Pref. Advanced, I change the burning preferences from Audio CD to Data DVD.
Then I create a new playlist and drag the whole Library into the playlist and click burn.
This will not backup all the meta data, or album art, but at least all the audio files are stored on one or more DVDs.

iPhoto Library Backup

To backup an iPhoto Library is much easier, I just needed to burn my iPhoto Library Folder to a DVD, which fortunately did fit on a single disk.

The other folder that is worth backing-up was my user folder of course ...
Make a backup once in while, you will be glad you did!