S3 Browser by Olivier Gutknecht seems very interesting:

S3 browser is a Mac OS X administration tool for the Amazon S3 data storage service. My goal is to build a small application useful in itself for developers or users subscribed to the S3 service, but also to provide example code showing how to access S3 through the REST API in a OS X Objective-C client application, on top of OS X technologies (Keychain, URL loading system, …).

[via Brent Simmons]