Danbooru uses Amazon S3 as a backup store for everything on the site. For backup purposes S3 is fairly cheap and reliable. There are no access restrictions on this S3 account because I want to easily download from it.
In July, usage of the Danbooru S3 jumped up significantly, peaking at 30 gigabytes a day on some days. For serving images, S3 is not cheap. My July bill was about $80 higher than average.
If you are downloading from the Danbooru S3, please throttle yourself to 1 gigabyte a day, or download from donmai.us. Or if you have any ideas what might be causing this (like a stupid code change I committed recently) please let me know.
I can't easily make the Danbooru S3 files private again, but I can enable logging to track referrers.
Updated