Interarchy (the program formerly known as anarchie) now has S3 support:
With Interarchy you can efficiently and reliably fetch, edit or SFTP/FTP transmit files to any kind of Internet server using FTP, SFTP, WebDAV, Amazon S3 or HTTP. Common uses include setting up web sites, long-distance transfer of data or remote server administration. Interarchy also supports other common Internet protocols including ping, traceroute, DNS lookup and packet sniffing. Because Interarchy is standards-compliant it will work with servers running on any operating system including Windows and Unix.
It's a $59.95 purchase via Kagi.
I wrote about Jungle Disk earlier, an Amazon S3 client for the Mac - Interarchy is miles ahead in the way of overall system maturity, since started out as an FTP client back in the olden days and expanded out from there.
Peter Lewis, the developer, wrote a tutorial on How Do I Make An Amazon S3 Hosted Website Using Interarchy - it's a start to finish description of using S3 as your backing store for a web server.