For those who don't already know, Google Voice, formerly GrandCentral is in beta and by invitation only. They're not handing out invites in the same way as gmail. Just leave your name and e-mail and wait a couple days.
Select one number to link your mobile, work, home phones, etc. You can also create settings for different callers (family, friends, co-workers, strangers) to ring different combinations of your various phones or to go to voicemail. Google Voice can also transcribe your voicemail and notify you via text and/or e-mail of a message received. Enjoy!