Usability 
GrandCentral (grandcentral.com/mobile) Google owned free call forwarding and voicemail service. I'm not going to describe the GrandCentral service in detail as it's been covered by thousands of articles and posts. Essentially it gives you a new phone number which you can set to ring any or all of up to 6 mobile, landline or Gizmo Project SIP numbers. There are loads of other features like call blocking and screening, custom voicemail greetings and temporary call diversion. GrandCentral's mobile web site gives you complete control over your GrandCentral number, you can change which phones your GrandCentral number rings, temporarily block calls or divert them to a new number, view call records, listen to voicemail by downloading an mp3 and manage and place calls from your GrandCentral phone book. The service has been in closed beta for what seems like forever but invites are pretty easy to get. In Fact I have 10 invites to give out. If you would like one just leave a comment.
Redirects to its successor, Google Talk.