If you are looking for a simple GUI for Amazon’s S3 servcie, there is a pretty good (free) browser plugin product available for Firefox:

S3 Firefox Organizer

Warning: The author of the product states very clearly that he is not responsible for deleted files. He also mentions that he may not continue to maintain the application. Of course, you can still access your files in S3 with another client if he decides to stop working on this free app.