For $100/year CrashPlan will store an *unlimited* amount of data and keep it for you in case of local failure of your storage devices. The excellent feature here is that they support Linux (rare for online storage offerings), Mac as well as Windows. If you plan to backup more than 100GB, they offer and recommend shipping you a 1TB drive with software to copy your data to the drive which writes encrypted & compressed, for mailing back to them.
In the event of a failure when you need to do a restore, you can order a drive to be mailed back to you (2 day priority) or download the data you need off their servers.
They also offer a nice little interactive table showing their pricing structure against other popular services where you can modify the amount of data you want to store to see how the pricing would change across different providers.
Crashplan offers a variety of plans that either count or don’t count the number of machines being backed up. Accordingly, plans vary from $4.50/month to $8.33/month depending on your needs. I have not yet tried the service, but I may consider it for backing up vital files to a secure off-site location.
I know of some folks who prefer services like Carbonite, but they don’t seem to support Linux. Another alternative, Jungledisk (front end to Amazon’s S3 cloud) has a Linux client for their backup server product, but I’ve never tried it. I have tried Jungledisk for Windows servers, and it works well, but Amazon meters the data being transferred to and from their cloud whereas Crashplan does not.