I'm a big fan of using Cloud based online storage for my images (DropBox, Amazon S3, etc).

Personally, I use JungleDisk to sync all my "keeper" images to Amazon S3 IN ADDITION to having multiple local copies on external harddrives.

Notes:

1. I only archive JPG's (not RAW images) online to keep the storage costs down.
2. I rate all images as part of my normal workflow, and then I export JPGs of all 1+ star images to the directory structure that gets backed up to Amazon.