When Google bought Grand Central three years ago, many Google watchers saw this day coming. Google Voice is now a free service for everyone in the U.S., no invite needed.

What is Google Voice, you may wonder? The fine folks in Mountain View have put together a pretty good two-minute elevator pitch:

[youtube=http://www.youtube.com/watch?v=cOZU7BOeQ58&w=600&h=370]

Google has been preparing for today’s launch by buying up literally millions of phone numbers from Level3. Those numbers will be taken by users who often choose to have nearby area codes (otherwise, Google could just start its own area code!) as their hub of voice routing.

Google currently has over one million active Google Voice users and had opened the service up to students on an .edu domain just last month.

What does Google Voice have in store for its users?  I see two major directions they are going with Voice that will be huge disruptions to traditional services.

1. Google Voice will be integrated into the Google Apps suite of services, allowing companies to add voice call routing and voice mailboxes to the Apps control panel (a huge boon to IT workers using Apps).   Game Changer.
2. Google purchased Gizmo5 VoIP service last year and will likely turn Google Voice and Google Talk into one big Skype-killing telephony engine. Recent rumors say that Google will forgo the Windows or Mac VoIP client and build a web client that will also be available on mobile devices with modern browsers. This will also have huge implications on the mobile industry, which has been charging for these services.

You can give it a shot at http://voice.google.com.

How big does Google think Google Voice will be?  Have a look at this graphic: