Encryption that is out of your control is not encryption in any meaningful sense. You must be the only one who has the key, else the whole process is compromised.
This is a pet peeve of mine, let me know if I'm being a pedant:
Encryption that is out of your control is encryption in a specific, meaningful sense. I believe to effectively use encryption, you have to understand the trade-offs involved and limitations of the technology.
So, Google's encryption is terrific in terms of protecting you from war-drivers. But it won't protect you from the focused attention of the FBI. That doesn't make it good or bad, it's a tool with specific uses and limitations.
I think that lesson needs to be absorbed with all forms of encryption. It's a particularly dangerous area to pop-sci oversimplify.
There are quite a few options available from the stand-alone, like SpiderOak, to things that sit on top of Dropbox, such as Boxcryptor.
What I haven't seen is a comprehensive security review of these alternatives. There could be bugs, flaws, or they could outright not be doing what they claim to do.
