Google talk (Windows, Mac, Linux) is a free alternative to Skype. It allows to make audio and video calls, create conferences. And it supports a text chat which is much better than the one offered by Skype.

You can use Google Talk software for making calls. And you can also use a Voice and Video Chat browser plugin that supports voice and video chat between Gmail users. The plugin is available for Windows (XP, Vista, and 7), Mac OS X (only on Intel-based Macs), and Linux (Debian, Ubuntu, Fedora, and OpenSUSE packages available, but the binaries work on other systems). The plugin must be downloaded and installed, but otherwise seamlessly integrates into the Gmail interface.