A quick perusal of my Contact page will show you the addition of this chat widget, for people who need to contact me, and don’t want to (or can’t) install something like, say, Google Talk. This is a good idea for people who are on ‘locked–down’ computers, such as in computer labs or in businesses with actual working security policies. My current state of online–ness is reflected here:
You can find out more from the Google announcement, or play with it after you log in to your Google Talk account or Gmail and start playing with it.
Limited usefulness
For those of use who can’t use the official Google Talk agent, when somebody clicks the widget, you’ll be alerted with this giganimus linked message:

That sends you to the same interface the the person who clicked the widget has, instead of moving you inside Gmail or letting you use your Jabber client. Thus, it’s really not that great of a help for people who want to use their own applications. But if you absolutely, positively have to chat with somebody — right now — this is as good a solution as any.

