While I have used Dropbox, I was wondering if there was a tool to sync files to Amazon S3 to gain the resiliency and reliability (though there would be a cost implication).
There is a service which allows you to access an S3 bucket with rsync, http://www.s3rsync.com/
They charge for the bandwidth in/out and an hourly fee (billed by the minute) for access to their rsync servers within Amazon's EC2. Being within EC2 you're not charged for the traffic between your S3 bucket and their servers but you will still have to pay S3 storage costs on top of the their fees.
You might want to check out Jungle Disk. You are paying a third-party a fee, but they offer a neat tool for synchronisation, backed by Amazon S3 and Rackspace Cloud.
I like Dragon Disk. It has syncing a lot of other great features.
It's free, and for tons of platforms including PC, Mac, and Linux.
You can read a review here.
You could use the software found at http://www.superflexible.com/. It is not free and is sometimes not the most intuitive to use - mostly because of the myriad of options available.
But it does support syncing to S3. It can be configured to run at set intervals or watch a folder for changes and you can run it as a service so it is mostly invisible.
I think ExpanDrive http://www.expandrive.com/ does exactly what you're looking for. I would also second the Jungle Disk recommendation - the added bonus being the integration with the iPhone app etc that ExpanDrive lacks. But ExpanDrive is a one-time fee, as opposed to a monthly cost.