Google Voice, or the service formerly known as GrandCentral, assigns users a single phone number that you can give out to friends, colleagues, enemies, or just about anybody else. You can then link that number to your existing phone lines so that when a user calls it will ring your home, work, and/or cellphone. You can set permissions so that calls from some numbers will always go straight to voicemail. And you can get text transcriptions of voicemails emailed to you.
There's just one problem - you have to give everybody a new phone number. To date there's been no way to make your current number the primary number for Google Voice or GrandCentral. But it looks like that feature could be coming soon.
In the Google Voice help section, there's an area labeled "Porting your number." Unfortunately, right now the page says that you can't actually do this... yet. But it does state that Google is looking into this option and the company is asking for feedback. So if you like the idea of using Google Voice but don't like the idea of reprinting your business cards, go ahead and send Googel a note to let them know what you think.