In case you hadn't heard, Google Voice is finally accepting sign-ups after almost a year in closed trials. I was lucky enough to get in when it was still Grand Central, before Google bought them and closed the application. For those living in caves, Google Voice is a phone aggregation (or replacement) service. When you sign up, you're given a Google Voice phone number. You can then register any of your other phones (cell phone, home, work, etc.). When someone calls your Google number, it will simultaneously ring all of your registered phones. It also includes a large number of other very useful features. And it's free.
http://voice.google.com
http://voice.google.com