There was a buzz when Google Voice first launched. That has settled down since their acquisition of GIPS and Gizmo haven't made it to GV yet. There have been copies, like Phone.com and others. Now there's an open-source version of it thanks to Twilio.
Thomas Howe has a review here, but I am borrowing his intro:
"Twilio announced their OpenVBX today, an open source Google Voice for business. Open VBX is a “modern, flexible business phone” platform, released as open source and works out of the box on top of the Twilio API. You can get the source yourself here, deploy it on any LAMP platform (PHP 5+), and off you go."
Howe thinks it will be an Asterisk killer because premise based boxes are not the future. Everything is moving to the cloud. Jobs are moving to the cloud, so the data will have to be out there as well - and so will the phone system.