Answer from 2014 for the people who are down-voting me:
Well, S3 isn't FTP. There are lots and lots of clients that support S3, however.
Pretty much every notable FTP client on OS X has support, including Transmit and Cyberduck.
If you're on Windows, take a look at Cyberduck or CloudBerry.
Updated answer for 2019:
AWS has recently released the AWS Transfer for SFTP service, which may do what you're looking for.