SugarSync is good, if more expensive.
SpiderOak is fantastic if you value encryption.

Now available for iOS and apple devices :)

I use three different services (all free I might add) for three different things! :)

Google Drive - 15GB (Photos)
DropBox - currently up to 5.9GB (Documents)
Box - 50GB (Music)

Grudgingly have to admit that my favourite is DropBox - I use Linux at work and have both a Windows desktop and a MacBook at home, so total cross-compatability is needed, and the lack of native Linux clients for Box and Google Drive is painful (but not unsurmountable).

Dropbox, for the most part when they don't have outages, "just works". A close second is Google Drive, and although Box seems to be making huge strides in their offerings (they used to just offer an "upload" feature without a "sync" app), the total lack of Linux support left me scrabbling around using Samba/CIFS.

Tried them all and this is the best by far
 

Try "Copy"; 15 GB for free, and you will automatically get 20 if you use this referral link below (disclaimer: I also receive an additional 5GB)
Use on Windows, Android, etc without difficulty; you my install a folder on any or all of your device(s) and in the cloud.
Believe me, they beat Google, Skydrive and Dropbox (and I have accounts with all of the above).
Enjoy!

https://copy.com?r=GsEX8J

I have absolutely no affiliation with this company, or any of the others referenced.

Syncplicity
 

I've been using Syncplicity for a few years. They were acquired by EMC a year or two ago. I found them a little more business like than Dropbox when I first signed up. Inertia has kept me there.

Google Drive.. integrates with Gmail and works pretty well.

I`m using 2 different providers.
Dropbox for local syncing between the laptops and table´s.
Amazon´s S3 and Glazier for the photos, which I just need to back-up. Mainly all photos are on external disks, but this is so cheap that I stored everything there in case something happens (dog eats the drive or similar)

Dropbox has been working fine for past 3 years, only once it deleted some files, but with re-store option everything was saved.

The extra free 5gb required me to download the Mac app. And iPhone/iPad apps on iOS also available.

So far looks good as extra storage space....^^

How are you interfacing with Amazon's storage services? Are you using a third-party client program?

I use Goodsync for this - works extremely well. Several hundred Gb of photos in S3, with a policy that converts these to the Glacier storage class after 1 day.

Also has two factor authentication which I like.

Google Drive is very stable.

I use the apple "iCloud" for photos only as it doesn't seem nearly as stable.

SpiderOak ...... all files encrypted on the host ......

I actually use the old fashion way and upload those via the Amazon internet page. I zip all files what I put there in one file and then upload that. I took one program to use, but this is not so difficult either. Of course if you are doing the uploads often, this is not so fast and easy solution.

For me this is 100% back-up, so if I really need those, then I will just download the whole zip file. Files are per year and topic.

I have also the Glazier, I think I put that running automatically after 30 days.

Back to the topic, I downloaded Cloudberry Explorer (free version) and it seems pretty easy to operate, just pull from folder to folder and it uploads on the background.

There is also option which you can use for making the the scheduled backups and you can see the used capacity easier than from Amazon´s statements.

I love, love love Bitttorrent Sync

It's free and you can make your own cloud kind of.

Much better in a way.

And it runs very well on all platforms.

Check it out at http://www.bittorrent.com/sync