Peter Krough has written an excellent article on backup in which he suggests the 321 system. It says that you should have three copies (one primary and two backups) on two different media with one off-site. Not only is this incredibly easy to remember, it really helps me sift through the myriad of backup solutions to get what I need and not a lot more.
For my on-site backup, I have mentioned before that I like to use Chronosync to automatically backup selected files whenever I mount a specific hard disc or thumb drive though Apple’s Backup (free with Mobile Me) or Time Machine will meet most mac user’s needs. On the PC side of things, I have heard good things about Paragon Drive Backup and NovaBackup.
For offsite backup, I use and am really satisfied with Carbonite while Chris has written a bit about his preferred offsite backup service: Mozy. Both are great and both support Mac and Windows. The point is that you need to have your mission critical files and irreplaceable memories (photos, videos) stored somewhere that fire or flood will not cause you to lose money or memories.
I think Peter Krough said it best in his article, “There are two types of people, those who have experienced hard drive failure and those who will.” Do yourself a favor and get your 321 system in place today.