The GoogleTalk plug-in? It's the light blue box above (or below) your
green label's box on the left of your screen (assuming you're using an
approved browser and have one of the "chosen" languages as your
preference, and are not using "standard without chat"). Titled your
Quick Contacts, it's an interface into the GoogleTalk (GTalk) system,
but it's not an independent chat client, since it depends on a number
of factors.
If you meant where do I find a Google Group that talks about
GTalk/GoogleTalk, then perhaps this search is what you're looking for
<http://groups.google.com/groups/dir?lnk=srgmtg&q=%22google+talk%22>.