Some of you might already have been using Google hangout, the online face-to-face chatting tool.
I think Google hangout can be used as a live online teaching/learning tool because you can present learning content to others while discussing online.
Note that you can chat with up to 9 people in Google hangout, though. So Google hangout may be better for small group discussions than whole class lecturing.
There are two ways in which you can present your content (for example, ppt slides) in Google hangout. You can share your screen itself or share slides using SlideShare app.
First, sharing screen is simple. When you start Google hangout, you can see 'Chat', 'Invite', 'App' and 'Screenshare' in the menu bar.
By clicking 'Screenshare', you can choose and share your computer screen with the people you invite. Whatever you have in your computer screen including power point slides and Youtube video will be on your friends or students' screens, too.
Below is the screenshot image of sharing a computer screen in the Google hangout. You can see a Youtube video is being played in the screen.
And if you want to use SlideShare app, you need to first upload your slides into SlideShare. Then after starting google hangout, click 'App' menu then choose 'SlideShare' app. And you need to search and select the presentation slides you uploaded to share with others.
Below is a screenshot image of using SlideShare app in Google hangout.
Visit here for more information for using SlideShare in Google hangout.