I've been waiting to try Google Voice since the service was announced a year ago. Initially was available by invitation only. A few days ago Google made it available to anyone with an US phone number.
The service is free, except for international calls, with very competitive rates (similar to Skype and Yahoo). Google Voice is not VoIP, you need a regular phone to make the calls, but it will connect you to your destination.
First you get a universal phone number, you can give it to your friends and business contacts, then you can forward your number to your office, cell phone, home, etc. This number can send and receive text messages like a cell phone, and you can forward them to your cell, to your email or read them in the Google voice interface.
You can also create calling groups from your contacts, and filter calls based on time, caller, Call ID. Also the system can ask your caller to identify himself before transferring the call to your numbers. You can also set some times of the day when you don’t want call to be transferred. It is like layers of privacy, you can change settings for your phones, individuals and groups.
When a call is made to your virtual number, Goggle Voice will try to contact you on all the phones, according with your preferences, and will answer the calls with integrated voicemail if you are not available. You can get a transcription of the voicemail message by text or email and you can listen to it online or calling your own virtual number.
Additionally you can add calling buttons on your website (look at the “Call Me” Button on the right, the system will ask for the number of the caller and name and will connect the two parties.
It is only available in the US for now, but more countries will be added soon.