Updated May 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 SugarSync but all the plans listed are great options.
If you find that a free online backup plan just won't cut it, be sure to take a look at my List of Online Backup Services for a complete view of your options. Some services even offer plans with unlimited storage: Unlimited Online Backup 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.
Cyphertite Basic gives you 8 GB of backup space for free.
Next to SugarSync's and SpiderOak's "friendly" possibilities of a dozen or more gigabytes (see their summaries below), Cyphertite's free 8 GB of online backup space is the industry's largest.
Open source software, easy setup, and super-simple web-based interface make Cyphertite an excellent choice no matter how much free space you're after.
2. 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.
Comodo Backup offers 5 GB of free online backup space.
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.
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!
SugarSync offers 5 GB of online backup for free.
One of the great things about SugarSync is that it's more than just a great free online backup service: it's also a really slick way to sync your data between computers and other devices like your phone.
Do you have a lot of friends that would also like some free online backup? If so, you could earn yourself as much as 32 GB of free storage. This is the most space I've seen given away, so far, as part of an online backup refer-a-friend program.
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.
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.
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.
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.
Other Free "Storage" Services
ADrive is another online backup service that offers a free plan: ADrive Personal Basic, with a monstrous 50 GB storage limit. However, the desktop application that automatically backs up your files isn't available, making it difficult to use as an actual backup service.
It's a similar situation with services like Dropbox, Box, Google Drive, and the like, which all offer free storage space. None of these services, however, are well suited to backup your important data.