Secret Message is a free application that lets you hide messages in plain text that you can later send to anyone via e-mail, social networks or IM. This nice utility is very simple. It consists of a single window, where you need to type two messages. First, there is the Plain Message, which is basically a distraction for anyone who intercepts the message. The Secret Message is hidden within the Plain Message, and it can only be seen when opened with this application.
Secret Message exports your messages in PNG image files. You can send them by attaching them to an e-mail message with your e-mail client. The PNG file will show the Plain Message if you open it with an image viewer. But, when you load it into this app, you can see the secret message by hovering your mouse over the "See Secret" button.
I tested Secret Message with a few different secret messages, and proceeded to export and then load the messages to see if the process works, and it really does. I don't think this is the safest way to send important messages, because there doesn't seem to be encryption involved, but it is a nice proof of the concept.