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.
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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.
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