Google Drive	OneDrive (SkyDrive)	Dropbox	IDriveSync
Free Storage Space	15 GB	15 GB	2 GB	5 GB (10 GB)
Additional Free Storage	none	500 MB per referral	500 MB per referral	1 GB per referral
Paid Options	N/A	50 GB: $25/year	N/A	150 GB: $50*/year
Next Step Up	100 GB: $60/year	100 GB: $50/year	100 GB: $99/year	500 GB: $150*/year
Even More Space	200 GB: $120/year	200 GB: $100/year	200 GB: $199/year	1000 GB: $300*/year
	Google Drive	OneDrive (SkyDrive)	Dropbox	IDriveSync
Concurrent Editing	Yes for online edit	Yes	Yes for Office 365	No
Link Sharing	Included	Included	Included	Included
Keep Deleted File Copies	RISKY!	No	30 days+	Yes
Security	SSL and encryption	SSL and encryption	SSL & AES-256 bit encryption	256-bit AES encryption

* Sometimes the IDriveSync site shows a 30-50% sale for "today only". In other words, the real IDriveSync pricing for "today only" is less than what appears above.
+ Dropbox keeps copies of files that you've deleted for longer terms if you use certain of their paid versions. Incidentally, IDriveSync keeps copies of your deleted files forever, unless you delete them manually from IDriveSync (in addition to having deleted them from your computer) or via their automated "Archive Cleanup" process.
! Google Drive is NOT a backup service and there are many complaints that G Drive has itself deleted files on PCs without the user intending such. For more info, read some of the posts at Google Drive Product Forums.

Encrytion is an option for these services. In some cases, e.g., with Google Drive, there is a cost to adding encryption.
Microsoft has gone all out to convince us that OneDrive is the best choice. An article with perhaps a more unbiased view comparing OneDrive, Google Drive, Dropbox and Box is OneDrive, Dropbox, Google Drive, and Box: Which cloud storage service is right for you?. You may also find Cloud storage made simple: How to integrate it with your workflow to be useful.

Although there are many other cloud providers, if you add iCloud to the products discussed above, we are looking at the products used by most people.

Some definitions:

• Concurrent Editing means that the system is designed to allow the same file to be edited from two different devices; e.g., from a desktop and a laptop at the same time.
• Link Sharing means that you can send a link to anyone who can then access the file that you wish to share with them from within your cloud storage.

To sign up for a free OneDrive account, go to OneDrive. By signing up from this link, you receive an additional 500MB of storage space over and above the standard 7 GB and I receive an additional 500 MB for the referral.

To sign up for a free Dropbox account, go to Dropbox. By signing up from this link, you receive an additional 500MB of storage space over and above the standard 2 GB and I receive an additional 500 MB for the referral.

To sign up for a free IDriveSync account, go to IDriveSync. By signing up from this link, you receive an additional 1 GB of storage space over and above the base and I receive an additional 1 GB for the referral. Each time I've clicked on that link recently, it offers you 50% off the pricing given above.