Here is a list of S3 clients that I have found, for my reference. Some I have used, some I have not yet. Some of these also work for more than just S3 (including SFTP/SSH).
GUI
- Transmit($34.00)
- Cyberduck(donate)
- DragonDisk(donate)
- Interarchy($49.99)
- Forklift($29.95)
CLI