Mozy and Carbonite but there are a few downsides (other than price). It can take a long time to upload, especially for an initial backup. Then if you do ever need to recover 100 gigs of data, it can take an incredibly long time to download unless you just pay to have DVDs of your data sent to you. I figured I could do better on my own.
When I got my new camera, I moved to using a single Western Digital Elements 2 TB external drive for backups. I kept it off-site but needed to either bring it home to update or use another drive to bring any updates to it. It wasn't good to have such a disconnected backup.
For my new system, I wanted to rotate drives. One drive will always be offsite and the other will always be at home, ready to receive files. Every few weeks I'll swap the drives, which means that they'll both be offsite for a while, but since my original files I'm backing up are at home on the original drive, that's pretty low-risk.
I started with a Diablotek EN3525D Hard Drive Dock and a pair of 2 terabyte drives. I went with two different manufacturers - Seagate and Hitachi - to avoid the possibility of getting two drives from a single bad production run.
I had been using Microsoft SyncToy to run backups but had some struggles with it. You need a sync definition for each drive pair and you can't run two in parallel or sequence. I switched to using a Windows command-line tool robocopy. That lets me build my own scripts and decide whether I want to run a quick update of just some files or sync everything.
One concern I have is that if any files on my original disks get corrupted I will end up replicating those corrupted files to both backup drives. Similarly, if a file is deleted from my original disk, I'll replicate that delete. I'm considering adding a third backup drive that I only ever add files to (contribute) instead of mirroring deletes. Or maybe it's time to look at a Blu-Ray burner?