Sending information over the internet can be like using postcards and VoIP (iChat, Skype, Gizmo, Google Talk, etc.) is no exception.
To solve this Phil Zimmermann, the creator of PGP, the most widely used email encryption software in the world, created an application for encrypting video and audio calls over the internet.
It’s called Zfone and it’s as easy as starting the app up before you begin a call (all people on call need the app). It works with Windows XP, Vista, Mac OS X, or Linux. The only VoIP app it doesn’t work with is Skype because Skype doesn’t use open standards, unlike every other VoIP app in the world, so you’re better off without Skype (use iChat, Google Talk, Gizmo or any other option out there).
Highly recommended, download it here.