Encfs

on osx
You will need brew and osxfuse installed. Once you have them installed, run the following in Applications/Utilities/Terminal:

$ brew install encfs

This will take some time, as it downloads all of your dependencies. Now to create your first encrypted directory. The encrypted data will be stored in .crypt, and when you mount it, it will be ~/crypt in your home directory. You could replace ~/.crypt with ~/Dropbox/Private to store your encrypted drive on Dropbox.

$ encfs ~/.crypt ~/crypt

To umount it, make sure you're not in the directory (~/crypt in the above example) and:

$ umount ~/crypt

You can mount it elsewhere later if you wish:

$ encfs ~/.crypt ~/newcryptdir

errors
You may get the error: $ encfs ~/.crypt ~/crypt dyld: Library not loaded: /usr/local/opt/gettext/lib/libintl.8.dylib Referenced from: /usr/local/bin/encfs Reason: image not found Trace/BPT trap: 5

If you get this error, install gettext and you should be OK: $ brew install gettext