Encryption for Dropbox or Alternative Cloud Storage Provider

[Skrambles]

What are the decent options for file encryption with Dropbox? I'm looking for something that is 'zero knowledge' i.e. I don't want the provider having my password or encryption key. I'm running a Mac (with OS 10.9) and various PCs on Windows 7, so need something which works seamlessly across platforms.

Are the following options any good?

Safebox - http://safeboxapp.com It looks very easy, but it's not open source and I can't find any detailed info about it.

Boxcryptor

As an alternative to Dropbox, what is Spideroak like? Is it as seamless as Dropbox?

[GlenMH]

Why not just have a truecrypt volume in your dropbox folder?

[Skrambles]

I have a lot of files to upload and using a Truecrypt container would make sync'ing quite slow because of the size of single Truecrypt file.