I've been looking into this for a week or two and trying out a few of them..........I've been using Jungle Disk with Amazon S3 for some time, and it works fine for me, but for some reason I'm driven to play around with the others.
(that along with an external hard drive have me feeling reasonable safe)
There are other interfaces available for S3, but I'm not aware of any others that do automatic backup.
FWIW, Jungle Disk was purchased by Rackspace, and though its not clear as yet what they plan to do, it looks like they are thinking of offering storage in both places.
I've also been using Windows Sky Drive:Windows Sky Drive
It has 5 GB free storage.
I have a few things stored on there and the interface is easy, but if you want drag and drop you need to use IE.
Windows Live Mesh looks interesting:Windows Live Mesh
After poking around, the biggest difference I noticed was that some offer file synchronization across one or more computers, some offer storage only and files are backed up manually, and some offer storage and back up, or storage, backup, and synchronization.
Most of them are accessible from anywhere, though some of the free ones limit you to one computer. Some of them will only allow certain types of files to be uploaded.
Dropbox has 2GB and is very easy to use:Dropbox
I have a handful of invitations if anyone wants one.
Dropbox stores their files at Amazon S3.
Syncplicity just came out of beta - I've been using it off and on. It hogs a lot of bandwidth.
Sugarsync has plans starting at $2.49/mo. US and will put your photos into gorgeous photo albums (so they say).
Most of the services allow files to be shared or kept private.
I'm leery of Wuala as they seem to store bits and pieces data on the computers of the people who join.
Maybe its secure but the very idea of my data being scattered about on other people's computers gives me the chills.
Lifehacker has a good and recent comparison chart of online storage
, and there are several more mentioned in the comments.