Google Voice is now open to anyone in the U.S. – no invitation is required. The free service provides one number to ring all your phones, voicemail transcriptions, and a ton of other things your carrier probably doesn’t provide. Check out the official Google Voice overview page for a complete list of features.
If you have never checked out Google Voice, I would suggest at least installing the Android client and see how it works first hand. The voicemail transcriptions are my favorite feature and the main reason I switched over from my carrier’s voicemail. Additionally, you can send SMS text messages using your Google Voice number without charges from your carrier. The latest update also allows for better inbox synchronization which allows for instant notifications.
Visit the official Google Voice site and just sign in with your existing Google Account to get started.