If you don't have a MobileMe account, select the Custom option at the bottom, give your backup plan a name like "Address Book, iCal, and Settings". Then click the plus sign ( + ) above the Destination and Schedule to choose what will be backed up. Select Address Book, Application preferences, iCal, and Safari Settings (this assumes that you use Safari). The application displays how much data it is backing up. For me, this was only about 28MB. If you find it is creeping towards 100 MB, you can create multiple, separate backup plans, each of which gets its own 100 MB quota. Finally, click the + button UNDER "Destination and Schedule" and choose where you want the data to be stored.
MobileMe users can back their data up to iDisk, while those not using MobileMe can backup to Dropbox or simply backup to a folder on your hard drive. On my iMac, I have my backups set to copy to a folder on my Drobo. On my MacBook, I have it sync to iDisk.
I highly recommend backing this data up every day. On my MacBook, I set it to start about 10 minutes before I need to leave the office to pickup my son from school. When I see the application launch, I start packing up whatever I need to bring home. By the time I'm finished, the backup has completed and I can pack up the laptop as well.
Why keep daily backups of this information? I don't know about you, but my calendar and address book information are crucial. I worry not only about data loss but data corruption, so keeping extra backups around seems like too easy of a step to not take it. Application preferences and Safari settings might not be considered crucial, but it's so easy to back them up, I can't see why not to add them as well.
MobileMe users should definitely checkout the options that Backup.app has for backing up iTunes and especially iPhoto if you use it. My iPhoto collection is probably one of the most important things on my computer, with 7 years of pictures of our son and vacations, etc. That data is irreplaceable. I wish Apple didn't limit larger backup sets to MobileMe users. There is no reason why anyone shouldn't be able to use this application to backup information to their own hard drive. Apple's decision to limit access to the full feature set of this application to MobileMe users is a completely arbitrary. In my opinion, Apple ought to be doing everything possible to encourage people to make backups.
One final "pro tip" regarding the Backup.app: by default, the application will wait for 2 minutes before starting. While it waits, it bounces in the dock. I find this to be a huge annoyance, so I tracked down a hidden preference which will reduce that to one second (and one bounce). Launch the Terminal.app from /Applications/Utilities in Finder and paste in this line:
defaults write com.apple.Backup 'Backup Timer' -int '1'
You can change the '1' to however many seconds you want it to wait. If you are thinking about choosing zero, please note that will actually disable the backup from running. One second is as low as you can go. 120 is the default.
At the risk of sounding Ballmer-esque: Backups! Backups! Backups!