http://safeboxapp.com
The app takes files it finds in ~/Safebox and encrypts them using AES-256 and stores the files in a subfolder of ~/Dropbox. When it sees new encrypted files in that folder, it decrypts them back to ~/Safebox. From then on, changes in either folder are automatically synchronized. The filenames and folder names are encrypted as well.
From a user perspective, you simply use ~/Safebox instead of ~/Dropbox for files requiring extra security.
It's packaged as a Mac OS/X app (and soon as a Windows app). We haven't decided pricing yet.
I'd love to hear your thoughts!!
Thanks, Denis
http://safeboxapp.com