Basically with Google Voice (GV), you have one new phone number that you can give out to everyone. When someone calls, it can ring all of your phones (i.e. cell, home and office). Or you can have certain phones ring depending on who's calling.
Record custom voicemail greetings depending on who's calling.
You can send, receive and search through all of your text messages online.
Probably the most fun reason for getting GV is it's an excuse to get a kick-ass new phone number. Mine is 502-PORN. =]
More features: About GV
Sweet, I got an invite! Now what?
Not gonna lie, I spent a solid two hours trying to come up with a good phone number. You can choose any number from any area code that's available. You can search by area code, words or numbers.