One of the main online storage players is Mozy. You get your first 2GB of storage for free and we use them at our office. Just to give you an idea of security of your content our office is under the watchful eye of the SEC (Security and Exchange Commission...the finance industry watchdogs...who didn't do enough for evil doers on Wall Street to easily slip past them). Our office is in the line of work that is the second most regulated industry in the USA and we got the "okay" from the powers that be to use Mozy...so it's secure.
The main problem with them however (which is true of all online backup) is it can take easily a month to backup your data (if you have large music/picture/video library). This is true only for the initial backup, after that then it only backs up your changes which are monitored constantly and have little impact.
Another program that I personally feel is better is Carbonite. They are pretty similar but I believe have a better pricing scheme, a better GUI (Graphical User Interface) and 2 excelent editions specific to them.
1. Carbonite uses a colored dot on your files and folders that are part of the backup selection. A green dot for completed backup, a yellow for pending.(I might be wrong on the color association...this is all from memory) The advantage here is that you can easily see what is part of your backup solution and what isn't which leads to...
2. Context menu integration (i.e. Right click menu). If you see something that should or shouldn't (based off your convenient dots) be in the backup you can right click the folder or file and change it.
There are only two dissadvantages that should be heavily considered with these products or anything similar.
1. Your backup and restore is dependent upon internet connection
2. Some solutions might constantly (in the background) use your internet connection to do what they need to do and if you are using a Mobile (think 3G) connection you can really run up your usage.
Hope this helps.
-Evan