Google has pulled off yet another spectacular with web applications... Yahoo have been trying to do this for sometime now, Ebay wanted to do this with Skype after their buy... but it can be said Google has reached their first with quality voice chat and video chat from a web app directly.
To use voice and video chat from Gmail, you have to install a browser addon called GoogleVoiceAndVideo (the size is pretty large: 447KB) and this enables Voice and Video chat from within Gmail, just like the current Instant Messaging (IM) window within Gmail. While you are checking your emails, you can chat from anywhere in the world. You don't need to have that client application installed.
Gmail had text chatting in the web app for a year or so and this helped gain some market share in the IM world. Google Talk and Gmail chat are still long way behind MSN Messenger and Yahoo Messengers usage, but continuous improvements like this may help Google gain some market share in the IM market...
The Voice Chat and Video Chat uses a proprietary technology called "Vidyo technology"... The quality is pretty good and is nearly same as the GTalk desktop application. Obviously like Gmail's chat, there are some bugs in Voice chat as well and sometimes the connection doesn't happen the first time. But most times the voice was clear and crisp. For a first day after launch the service was pretty good.
I often use Skype for Voice and video chat. Skype has got a Linux client and its voice is one of the best in terms of quality. Gmail's voice doesn't work with Linux. Both Skype and Gmail's voice does not use Session Initiation Protocol (SIP), which I would have liked to see, to allow inter-interoperability between different VoIP providers. Nevertheless, bringing it to the browser is a new improvement that will bring ease-of-use to the users.