Google just announced that Linux users can now finally use voice and video chat in Gmail. For now, this is an Ubunutu-only feature, though the company plans to add support for other Debian- and RPM-based Linux distributions (like Red Hat and Fedora) in the near future.
Until now, these features were only available to Windows and Mac users. For these platforms, Google introduced voice and video chat in November 2008.
To get started, Ubuntu users can download the required plugin here.