Dropbox does back ups very well.   Behind the scenes the small program runs while you are working and It backs up all my files and file changes changes on any computer connected to my dropbox account as well as keeping an online backup for accessing files anywhere.

I have a couple computers tied to my dropbox account that always keep all my files in sync.  Each one has a external hard drive that keeps the computer backed up.  So I have data backed up on each computer and on each external hard drive via Time Machine and in the cloud + I have iPhone access...so everything stays in sync in 6 places with each file change I make. 

You can also share files with other dropbox users and share files publicly for easy download.  Pretty amazing.  What else could DropBox offer?  If Dropbox integrated with 3rd parties like box.net does to allow you to do more with the files you've uploaded, then it would rule this space.