Jitsi is a multi-protocol, multi-platform voice and video instant messenger client. It is mostly implemented in Java, and is Free/Open Source software licensed under the LGPL.
Where other VoIP services and clients send data in the clear, Jitsi sets itself apart by making use of ZRTP to set up secure communication with SRTP, Secure Real-time Transport Protocol.
Installation on Debian-based distributions involves downloading and installing Jitsi’s .deb package from here. After you click on the package link, the window shown below should open. (For other platforms, visit the general download page here.)
Clicking OK on that window opens the package in the graphical package manager. In Ubuntu 12.04 and earlier, the Software Center, Ubuntu’s graphical package manager, should open. Click Install.
After installation, search for and launch the application from the menu. The main interface is shown below.
This image shows the supported protocols. This is why it is called a multi-protocol IM client. You can now do all your voice and video chatting from one simple application.
And this one, Jitsi’s security options configuration window. Encrypted communication from your Linux desktop.
Some users had problems with the last stable release. Those issues should have been fixed in this latest stable version.