s3sync: This is a ruby program that easily transfers directories between a local directory and an S3 bucket:prefix. It behaves somewhat, but not precisely, like the rsync program. In particular, it shares rsync's peculiar behavior that trailing slashes on the source side are meaningful.

s3cmd: This is a ruby program that wraps S3 operations into a simple command-line tool. It is inspired by things like rsh3ll, #sh3ll, etc., but shares no code from them. It's meant as a companion utility to s3sync.rb but could be used on its own (provided you have read the other readme file and know how to use s3sync in theory).

See http://S3Sync.net for more info.