MIT Technology Review recently published an article about ShadowCrypt. While it sounds like a new videogame, ShadowCrypt is a browser extension developed by university researchers. Using ShadowCrypt, a person who writes or is authorized to read a tweet or e-mail sees regular normal text. However, anyone else looking at it – including site operators or persons intercepting the post – see only a garbled string of letters and numbers as ShadowCrypt encrypts the message. While currently available for Google Chrome, ShadowCrypt is still only a prototype.
You can read more about ShadowCrypt here.