We have been looking at M$ azure blob storage for our larger backup files. They obviously won't ship you physical media if you need to restore quickly, but for our SLAs, it's not a big deal.
As they are not a specialized backup provider, their prices are quite competitive.
We haven't committed to using them yet, so I can't comment on how well this works in practice, but I thought I'd throw it out there as it had not been mentioned.