I have a MacBook Air with a very small hard drive.
So every night, I'd like to move all my large files off my hard drive and into S3.
What's the best way for me to do that?
Dolly Drive is the easiest way to accomplish this. It's like Time Machine but it can set to do its backups to an S3 share you own. Very nice software.
If you'd rather go a little more DIY you could use something like Panic's Transmit to mount an S3 bucket as a drive and then rsync the days in to the file share. Just run the rsync as a launchd job every night.