A difference with Tarsnap is that s3ql is free and open-source software (both the client and server), so you can also use it on your own server, your friends' s

A difference with Tarsnap is that s3ql is free and open-source software (both the client and server), so you can also use it on your own server, your friends' servers, etc. I used s3ql to back up my stuff on servers where I had shell access and disk space but didn't trust root. By contrast, from what I understand, tarsnap requires you to use the official server.