If you knew it as "SIP Communicator" before, they have undergone a name change to reflect the increase in scope of functionality from "just SIP" into "communications powerhouse." They really describe a lot of audio, video and screen-sharing functionality built-in.

The interview is quite interesting as it covers a wide range of applications for a full-featured, Open Source client such as Jitsi.

Also, FWIW, I was genuinely surprised by how well it ran and looked on OSX, given that it is a Java app.