Google introduced a new feature in Gmail. The ability to call a landline within Gmail for free in US and Canada and at a very cheap rate for the rest of the world. Like Skype, there will be a dialpad for you to make the call out.
To use this feature, you will need to install a plugin.
After restarting your browser, you will be able to see the Call phone sign. Click on it will prompt you the dialpad. You can increase the credit if you want. By default, it already has US$0.10 in it. To call to Singapore land or mobile line, the rate is US$0.02
I did a check with Skype, the rate for Skype call to Singapore is US$0.017 + connection fee which is almost the same as Google Voice call.
Based on Engadget, it seems that Google is using the technology of Gizmo5 that they had acquired for this feature.
I have not tried the service personally. Here is a video of Gmail Voice in cartoon.