There are a lot of great online backup tools out there, one of the biggest names being Carbonite. I set up storage for another company recently which only required about 2 GB of data to be stored off-site (accounting files) and was able to install eDrive on their Windows Home Server computer which automatically backs up the data off-site daily and for up to 2 GB was a completely free account. eDrive is one of the few online backup programs that can run on a vast number of operating systems including Windows server environments and I believe even Linux, where Carbonite was only able to run on your standard Windows XP/Vista/7 computer.
Just keep in mind, however, that most of the time online backups are not feasible for all your customer art files which can be enough to fill an internal hard drive. Not only does it take days (depending upon your internet speed) to upload the changed or new files to their servers, but the cost for an account of such magnitude can be more than a brand new internal hard drive per month. If you are using a smaller account to backup smaller files, such as your accounting files, then it is much more feasible. We use the online backup that comes with Quickbooks and have up to either 1 or 2 GB of storage for free with our yearly support package.
For those of you doing backups, I'm curious, do you backup individual files and folders from one location to another drive (so that all the original files and folders can be viewed and recovered directly from the backup drive) or do you make disk images or clones of your original hard drive and store the image on a second drive (so you must use a disk recovery utility to recover the files)?