I've been reading through the Amazon S3 developer documentation tonight, and I've gotta say I'm quite impressed. This is a perfect application for a REST API, and they sure seem to have nailed it. I fully expect that as the available APIs built on top of it mature it'll start getting built into all sorts of interesting applications.
I'd love to see a library for using this from Mozilla plugins. Seems tailor made for persistantly storing data that needs to be accessible from multiple machines, If-Modified-Since and ETags would let you avoid wasting bandwidth downloading stuff you've already seen, and Mozilla's already got all the infrastructure you'd need to do the http-fu.
The other main application I'm seeing for this is encrypted offsite backups for people who are way too lazy to maintain their own storage. Fortunately, it looks like Brackup has that use case covered.