From Google blog today
Gmail voice and video chat will be rolled out globally over the next day or so for Macs and PCs. The first time you use this feature, you'll be prompted to download and install a small plugin. To get started, visit http://www.gmail.com/videochat or click on the "Options" menu in a Gmail chat window and choose "Add voice/video chat."From viewing videos on YouTube to screening family events, people love being able to watch something exactly as it happened. And as webcams have become popular, more and more of us are realizing that video is the next best thing to an in-person conversation. Today we're introducing Gmail voice and video chat, which lets you have free voice and video conversations right from within Gmail.
Video chatting from Gmail is as easy as sending an instant message. With our team spread out across Google offices in Sweden and the U.S., it's been really handy in helping us work together. Just click on the new "Video & more" menu in a Gmail chat window and select "Start video chat" or "Start voice chat." You can switch to a full screen view or pop out the chat window and change the size and positioning as you wish. Of course, not everyone has a webcam, but even if you don't, you can still have voice conversations alongside your email and regular chat.