Google has recently started giving Google Voice accounts through invites. Google voice allows you to have one Google-Voice-Number and associate all your other phone numbers to this global number. Then you can receive and make calls using this number without letting your callers worry, where you are calling from. You have more control over your number, you get a big bundle of other features including conference calling, call forwarding, conference call and the list is never ending. You can check out the list of fetures at Google Voice Website.
Now Google has also released a mobile-app for BlackBerry and Android handsets that allows you to make and receive calls, check your voice mail and do a lot more. You can download the mobile app from http://m.google.com/voice.