In November 2009, Google acquired Gizmo5, a powerful VoIP telephony service that allows users to place and receive calls from their computer and mobile phone applications. Today, the company has started informing users that it will be shutting down Gizmo5 service on April 3, 2011.
Gizmo5 offers desktop applications that users can leave open 24/7 without having to worry about keeping a browser tab open to receive calls. Google was also working on a desktop client for Google Voice based on Gizmo5 technology, but that project has been scrapped in favor of the Gmail/Google Talk browser plugin.
Here's the email that Google is sending to users: