There are many options for backing up your storage on-line.  The term "cloud computing" refers to companies that provide services over the internet, for example, Google Apps or Google Docs.  Storage is another area that has really expanded. Using off-site storage is an excellent was to ensure that in the case of flood, fire or other catastrophic event, you can restore your documents.

Mozy - automatically archives or backs up files that you specify over the internet.  Backups can be performed during your down-time or as files are changed. Cost as of writing this is $5 per month (as of March 2009). Recommended by PC Magazine as one of the 25 Best Business Software Tools but applicable to anyone with storage needs.

 DropBox - provides storage and syncs over any computer on which it is installed