Updated December 2013
Several backup services offer free online backup plans. The only catch with a free plan is that you're heavily restricted, at least compared to their premium offerings, as to how much data you're allowed to backup.
Aside from a relatively small amount of storage, a free online backup plan is usually identical to the not-so-free plans offered by the same company. This means that as long as a free plan has enough storage space for your data, and meets any other criteria you have, you can have a complete and permanent backup solution for free!
Below is a list of every free online backup plan that I could find, sorted by the amount of free online storage offered. I've had the best overall experience with Bitcasa, but all the plans listed are great options.
If you find that a free online backup plan just won't cut it, see my List of Online Backup Services for your not-so-free options. Some services even offer plans with unlimited storage: Unlimited Online Backup Plans. See my Business Online Backup list for business-class plans.
Important: The free online backup options below are truly free. I have not included any trial or temporary plans. See my Online Backup FAQ for more information.
Please let me know if anything below needs updated.
Comodo Backup offers 10 GB of free online backup space. Permanently. No joke.
Next to Bitcasa's "friendly" possibilities of dozens of free gigabytes (see their summary below), Comodo Backup's free 10 GB of online backup space is the industry's largest.
Comodo's interface is really straightforward and easy to use, which is something I can't say about the desktop software component of most online backup services.
Cyphertite Basic gives you 8 GB of backup space for free.
Open source software, easy setup, and super-simple web-based interface make Cyphertite an excellent choice.
3. MiMedia Free
MiMedia's Free plan offers 7 GB of free backup space.
Real-time backup, online music, photo, and video players, and easy file sharing make MiMedia a pretty solid online backup option, free or otherwise.
AVG LiveKive offers 5 GB of free online backup they refer to as their Starter version.
LivKive by AVG has an easy interface, support for unlimited devices (this is a rare treat) and support for all major computer operating systems.
Note: Make sure you sign up for the right plan if you're interested in the forever-free 5 GB. AVG also offers a a free trial of their Unlimited plan but this isn't the 100% free plan. On their website, click on Backup Plans and then look for the 5GB Storage free button under their plan comparisons.
5. IDrive Basic
IDrive's Basic plan grants 5 GB of free online storage space.
As like most of the other free backup offerings, you get to enjoy all the features of IDrive's premium offering: IDrive Pro.
One special bonus with IDrive Basic: unlimited device backup. In other words, you can backup all of the computers, smartphones, and tablets in your home that you wish, all to this single account. Just keep the total under 5 GB!
6. Bitcasa Free
With Bitcasa Free, you get 2 GB of backup space for free, with a maximum of 20 GB with friend referrals.
Excellent mobile access, drive-like access in Windows and Mac OS, and very easy folder syncing make Bitcasa a very good choice, no matter how much space you're after.
SpiderOak works a bit differently in regards to free online backup space. With SpiderOak, the first 2 GB of online backup space in any account is free. If you choose to go over that amount, you'll start to incur charges.
They also have a refer-a-friend program which earns you an additional free GB of space for each friend that signs up. The maximum reward is 10GB, meaning that you could snag yourself a relatively huge 12 GB of free space... so long as you have good friends.
Mozy offers 2 GB of free online backup in their MozyHome Free plan.
MozyHome Free uses the same software and sports the same features as their premium MozyHome plans, minus only the live technical support.
Moazy also has a friend referral program that could earn you 5 GB of additional free space, for a total of 7 GB.
ElephantDrive's Lite Edition plan offers 2 GB of free online backup space.
The Lite Edition has all the great features that ElephantDrive's premium plans have including military grade encryption, support for up to three computers or devices, and of course automatic backup.
MyOtherDrive gives away 2 GB of free online backup.
MyOtherDrive's free plan does not offer the premium offering's "file linking" capabilities and you may also see some advertisements within their program.
Other Free "Storage" Services
ADrive is another online backup service that offers a free plan but the desktop application that automatically backs up your files isn't available, making it difficult to use as an actual backup service. SugarSync also has a free plan but it's limited to 90 days of use.