My early-stage startup just left stealth mode to enter closed-beta. Our product, nCryptedCloud (which currently works with OSX 10.7+, Windows and iOS) is tackling the problem of data privacy and secure collaboration in the cloud. We're starting with Dropbox first, other cloud storage providers are in the plan.
nCryptedCloud is an enhancement layer on top of Dropbox. It encrypts & decrypts your Dropbox files seamlessly. Your files are always protected and you control who get access to them. For collaboration, you can revoke access to shared folders any time. Yes, even if they have the file, without permission, that file is not readable. Our purpose is protecting data at rest and data in the cloud.
Please give it a try if you have Dropbox installed. I would love any feedbacks you may have. Of course, it's free: http://ncryptedcloud.com/?rfc=VN-FRIENDS
Thank you,
-V.