The new S3 Web service has what I would consider a very well designed REST interface that is also well documented (all the Amazon Web services have decent documentation). The current set of Amazon Web services is starting to grow nicely.
The S3 service is advertised as a Web-scale storage system with a super simple API and apparently very low storage and bandwidth costs. Each resource stored can be up to 5GB and also has support for ACL based permissions - to access the ACL, just append "?acl" to the resource. That's pretty simple and effective.