Arq

Arq is a simple Mac App that will let you back up anything to the dirt cheap Amazon Glacier service. Pricing for Glacier is a bit complex (you pay by exact usage), but for most use cases you will end up spending less than 1/10 of what you would pay for Dropbox or similar alternatives. Backing up to the pricier Amazon S3 is also an option.

I've been using arq for about 2 months now. I push my backup on a server using sftp. I think it's the perfect complement to time machine on a local hard drive.

For windows: I'm a happy user of Cloudberry Desktop Backup (http://www.cloudberrylab.com/). I use S3, but has support for Glacier and most of the other data providers.

Excellent product. Regular improvements. Good customer service. Let's you backup to Google "Nearline" which is fantastic for low price point.

Arq is the real deal.