Too bad this service is only available in the US (for now...)
Google Voice provides you with one number. One number that you can use to forward to anywhere you want, any phone you want. You still need a phone service, but now you can move to a different city and not have to worry about giving out your new phone number - u just gotta change where your Google number is forwarded to. You can listen to voicemails online or you can read a written transcript of them! And did I mention the free long distance calling??