I've been using google voice for about a month now and I'm impressed. Voice (http://www.google.com/googlevoice/about.html) is Google's answer to integrating the web and voice services. After selecting a phone number from Google's database you can assign numbers so that a single number will ring multiple phones. In my configuration I have my Google Voice number ringing my mobile, my home, and the built in number in my vehicle. One number to rule them all.
Also text messages can be delivered to multiple phones and Google host your voicemail box and a wide variety of options for call screening and blocking. By far the neatest feature though is the voice transcription. Voice listens to your callers voicemail and then translate their spoken words to text. Then sends that message to you via text. I was extremely and pleasantly surprised at how well this works. It also notifies you via email.
From the voice homepage you can view all of your past text messages and voicemail and even listen to your voicemail from your PC. You can also send SMS and dial numbers. Voice basically treats messages and voicemail like email messages and even integrates your GMail contacts. If its this good and still in beta we can't wait to see what the final released version will be like.