Windows/Mac: Are you worried about security in your Dropbox account but don't want to go through the extra effort of encrypting everything on your own? SafeMonk is a solution that encrypts your files automatically.
When you install SafeMonk, it creates a folder in your Dropbox, and everything in that folder is encrypted. Before your files are uploaded to Dropbox, SafeMonk encrypts them so that they require a key to unlock. Without SafeMonk installed on your computer, you can't get access to your encrypted files.
It's not exactly foolproof, SafeMonk works well as a simple way to protect your files without a lot of work.