Ekiga is an open source SoftPhone, Video Conferencing and Instant Messenger application for Windows and Linux.
Many of us wonder, why is there so much excitement over Skype when, at its core, Ekiga is a more robust application with even more maturity behind it? The answer to that is rather simple - provide a compelling reason to use something besides Skype. Honestly, I believe Ekiga blows Skype out of the water with mature features (video calls) and it's open source heritage, which means that nothing 'weird' is going on without you knowing about it. Yet many Linux users by the truckload will still migrate over to Skype because they 'know' about this application from other platforms.