Overview. I just discovered a new cloud service called Copy, which works like Dropbox: you install a program, you place your files in a folder and they will be automatically uploaded in the cloud and synchronized with other devices using the program. Sharing a folder with friends works similarly (you can share a URL or send an invitation by email).
Mobile. Copy for Android is available, as well as Copy for iPhone. You can get your photos uploaded automatically on Copy. As Copy is a new service, Dropbox still has the leadership as many Apps let you use it, but don't know yet about Copy. This will change in the future, but just keep this in mind for now.
Storage. Comparing to Dropbox, which only offers you 2 GB to begin, Copy lets you start with 15 GB (!) of space, but if you use a referral/invitation link to sign up both the inviter and the new subscriber get 5 GB of bonus!
So, just subscribe now, using my referral link, install Copy and start with 20 GB of space (enjoy!): https://copy.com?r=OlWTYx
How to increase (for free) your available space. After installing Copy, you can also invite your friends. For each referral you get 5 GB bonus (Dropbox gives you only 250 MB for each referral). Lets do some math:
With Dropbox you invite 10 friends. You get 10 * 0.25 GB = 2.5 GB of additional space.
On my free Dropbox account I have only 10 GB of free space, which is not enough to backup my entire collection of pictures...
With Copy you invite 10 friends. You get 10 * 5 GB = 50 GB of additional space.
With a few invites, thanks to Copy, I can backup my entire collection of pictures and all my important documents, all for free!
Invite 10 friends and you have in total, 70 GB of free space.
Dropbox -at the moment of writing- has a special offer for Smartphones such as the Samsung Galaxy S3. When configuring your phone for the 1st time you can create a new account and get 50 GB of free space, but only after completing 5 steps in the "Getting started" and only for 1 year (I tried the promotion and I didn't get the 48 GB... quite disappointed of Dropbox!). After 1 year all the free space (48 GB) will be removed and you still have 2 GB of space.
See also my article Copy vs. Dropbox: Features comparison.
Conclusion. If you are looking for a FREE backup solution, sign up now to Copy and invite as many friends as you can. The 5 GB bonus for each referral, as indicated in their website, is a limited promotion till Copy is famous enough. Later, probably, they will lower the referral bonus.