I struggle with this issue as I have a bunch of computers.
I am giving dropbox a try, but I will only use it for a small set of useful files, not for my whole file backup/sync strategy. I don't want to pay for their premium service to back up all of my photos and videos etc etc.
I have been using 2Brightsparks' software called syncback, which is free. It is pretty flexible for backups and syncing.
I am also going to give "synctoy" a little bit more time for usage and testing.
I think my strategy is going to be to have 1 drive (external) be designated as my master drive. I will then push changes (adds, changes, deletes) from the master down to the backup drives.
It's going to take a lot of work to get to a perfect state, though.