The most basic services are those, such as Mozy and Carbonite, that specifically focus on backing up files from your main machine. Such services typically offer ‘unlimited’ online back-up for your files for a flat subscription fee, so you could theoretically back-up the entire contents of your computer’s hard disk, including the entire operating system.
In practice, though, you’d be unlikely to back-up more than a couple of hundred gigabytes using current broadband speeds - if you do, Carbonite warns you that your initial back-up "may take several days" to upload, for example. Even so, that would still allow many users to store all their crucial files for barely a fiver a month, which is good going for reliable and theft-proof off-site storage.
Review: Amazon S3
The 5GB "free usage tier" that Amazon offers to users of S3 – short for "simple storage service" – might seem to put it alongside the free offerings from the likes of Mozy, Dropbox and SkyDrive. However, S3 is a different kettle of fish altogether, and is very much aimed at big business users who need many terabytes of secure, online storage.
It’s not bad value for money, with prices starting at around 9p per gigabyte, but the rather complex pricing scheme means that you also have to take into account the total amount of data traffic that passes through your online ‘bucket’ each month.
Even more complex is the system of credentials that are required before you can upload even a single file. That level of security will please Amazon’s corporate customers, but it left me in a state of utter bewilderment. In the end I was only able to use S3 with the help of a Firefox add-on called S3Fox Organiser, which provides a more user-friendly front-end for the service.
Reg Rating 50%
Free Storage 5GB
Extra Storage Variable
More info S3
Next page: Off site but out of sight
Do these 'free' services help themselves to a lifetime worldwide licence to use your data?
Can you be sure data you've deleted is gone?
Is my data encrypted?
What assurances do you have over availability of your data?
If the company goes bust does the liquidator inherit my data?
If the company goes bust can I still get my data?
Where do the back ups go?
Is it legal for my data?
Before you upload your office data to those services have you considered the impact on your companies policies and legal requirements?
Are you transferring personal data out of the EEA and therefore breaking the DPA?
There are so many questions you have to know the answers to before you use these services. It's not simply a matter of how much is it and how easily can I spread my data.
from a home user's perspective...
Will amazon/apple/microsoft/et al be trawling through my files to try and sell me things?
Will I find my university thesis/latest novel in progress/etc available via Amazon if i search?
Will i find my holiday photo's used in Microsoft's latest marketing campaign?
whilst other people getting at your data is one thing, how trustworthy are the people you are handing it to, and what access are you granting them in exchange for this "free" storage?
I agree, there is a lot of questions that *everyone* should be asking of on-line providers, and I guess the above list covers the key ones. In fact, the simplest of all is this:
Do I have the encryption key, and it is not known to the storage host?
For most other factors, where the data is held, what happens if they are bought/liquidated, etc, they become less important as they cannot DO anything with my data as they don't get it plain-text.
OK, you have to ask what happens if it vanishes, but again you must look at the 'cloud' as a good HDD, not as a complete solution. You should have 2 copies of your data no matter what! 
And I know the arguments about de-dupe, but surely you could have a user-side client encryption that is block de-dupe friendly by encrypting blocks of the same size (4kB, 64kB?) with the same key-based pattern so they still de-dupe even if the plain text is unknown?