BeInSync offers 1GB for free, which I use for my essential documents and smaller files. As the name suggests, it also keeps such data in sync over multiple workstations.

I voted for Amazon S3 as that's where my data lives but I thought it worth noting here that the client I use to do the backup is Jungle Disk. It's a one-off charge for the software license and then you just pay Amazon's incredibly low storage charges directly to them.