Well, Photoshop.com offers online backup and storage. You get 2 gigabytes free, and you can purchase up to 100 gigabytes of storage space.
The advantage is that you can set up which files you want backed up in the Organizer and it will back your files up automatically as you work, without interrupting your work.
You can also find any number of inexpensive backup sites by doing a Google search on "online backup".
You'll see lots of results, so don't be afraid to shop for the best price. Here's a PC Magazine article recommending a few.