Like everybody else, I do multiple backups, but I've taken to using
JungleDisk as my preferred offsite backup system for several reasons:
1. I'm paying for it, which means that it's much less likely to vanish unexpectedly.
2. I can do password encryption on it. Which is mostly important because I'm paranoid. But since I'm paying for backups, I might as well put my taxes in there as well, in which case I really do want password encryption.
3. It runs on Windows, Mac OS X,
and Linux, and up until the hard drive in my Windows machine died on me, I used all three.
It's $30 for the software, flat fee, and then $0.15 cents per *gigabyte* of data transferred per month. Since I do most of my writing in plaintext, you can imagine the size of my monthly bill. The Amazon S3 cloud was set up for businesses, really, but it's nice that they let normal users take advantage of it. And you can set it to back up automatically, in the background. (You can also throttle the connection so that your normal internet activities don't suffer too badly.)
So, if you're looking for something a little more secure, it's an option. (There's a lot of other options that back stuff up to the Amazon cloud, btw, I just find JungleDisk's user interface to be very simple.)