Some time ago I wrote an article: Dropbox vs Copy: Copy offers more free space and after some testing and daily usage, I prepared this comparison table.

Feature	Copy	Dropbox	Comments
Desktop client
Windows, Mac, Linux	Yes	Yes
Selective sync	Yes	Yes
Pause sync	Yes	Yes
Bandwidth Limit	Yes	Yes*	Dropbox has an additional option: "Limit automatically" for upload (I don't know how it works)
Manage shared folders	Yes	No
Web client
Search Function	No	Yes	Sadly, Copy has not yet a search function (which I think it's very handy in Dropbox), so you will have to browse all your folder structure to find what you need...
Picture Gallery	Yes	Yes
Video Player	Yes	Yes
Download entire folder as Zip file	Yes	Yes
Move files into another folder	No	Yes
Versioning	Yes*	Yes	*In Copy.com you have to click the file, then on "File activity" to see the older versions. In Dropbox it's more user friendly: you click directly on "previous versions" and you can directly restore to an older version (which is not yet offered in Copy.com)
Restoring deleted files	No*	Yes	*Copy.com doesn't allow yet to undelete files via web client. Recover deleted files with the desktop client
Sync speed	Fast	Slow	Maybe it's just my impression, but Copy seems faster than Dropbox.
Sharing
Share a folder and let people add/remove files	Yes	Yes	Invitation sent by email
Copy a shared folder to your account	Needs a few minutes	Instant	Copy seems buggy: it creates a folder and it's empty. Only a few minutes later you will see your files... so don't worry, be patient.The button "Save to Copy" is available only when you are logged in and it is displayed at the bottom of the page, like in this screenshot:
Space used when sharing	Shared between peers*	Counted the same for each person	*With Copy, if you share a 2GB folder with 3 friends, "Copy" will count 500MB (2GB/4) for each person. With Dropbox, every person will lose 2GB from his/her account
Storage
Free account	15 GB, 20 GB with a referral	2 GB, 2.5 GB with a referral
Maximum free size	Unlimited (!)	18 GB :(	With 8 people using my referral link I already have 60 GB (thank you!)
Bonus / referral system	5 GB (!)	0.5 GB :(

Feel free to comment. I may update this table as soon as I discover new features or relevant differences.

Conclusion: it seems to me that Copy.com is the most generous service available (unlimited space offered for free, and 5GB for each referral). I just canceled my crashplan account and didn't renew my flickr pro one. Why paying when I can get my backups hosted for free?

However, if you need a better support for versioning and undeleting files, I recommend you to use Dropbox for that. I use both. Dropbox for my little projects (source code doesn't require a lot of space) and Copy to save my entire collection of pictures and videos, which needs a lot of space.

Check also my post on How to use Copy to backup an external hard drive.

Note: I will remove the referral links in the comments, so please do not put them. Thank you.

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.