If you go to GMail Chat and try to do a Voice and Video chat, you should be prompted to install the plugin. Sadly, it gives you a .deb (Debian (and thus Ubuntu)) package to install with. If you're using Fedora, you can follow these handy instructions.
I ran into an issue with SELinux,
[3695:3695:2615825547:ERROR:base/native_library_linux.cc(28)] dlopen failed when trying to open /opt/google/talkplugin/libnpgoogletalk.so: /opt/google/talkplugin/libnpgoogletalk.so: cannot restore segment prot after reloc: Permission denied [3695:3695:2615892549:ERROR:base/native_library_linux.cc(28)] dlopen failed when trying to open /opt/google/talkplugin/libnpgtpo3dautoplugin.so: /opt/google/talkplugin/libnpgtpo3dautoplugin.so: cannot restore segment prot after reloc: Permission denied
As I posted in a comment at the link above, I overcame it with this:
$ chcon -t texrel_shlib_t /opt/google/talkplugin/*
I have only used it for voice so far, as I don't have a webcam with me. I used it with Google Voice's new free calling to the US and Canada. Something I didn't realise, though, was that it doesn't yet have an option for a dial-in number if you do not have a Google Voice account, in which case you can still call out and you'll show as the number 1 760 705 8888 or something like it. (18.104.22.168 is also the IP address for one of Google's two public DNS servers.) I am eagre to have a Google Voice number, once availability exists in Canada.