Google Voice was previously GrandCentral before Google bought them out. We love it. We actually have three GC/GV numbers. One we use for our personal number, one I use for business and one we give out to places that are likely to make calls or sell off for telemarketing.
A Google Voice number provides you with a central number. No need to hand out your cell phone number. If someone calls your GV number you can have it ring your cell phones and home phone simultaneously. In its default configuration Google Voice will intercept the call and ask the caller to state their name. If you have placed them in your GV phone book already it will skip the request to state their name. It will then call your phones. When one or all of the phones are picked up you'll get a greeting "Call from Mike Smith" and gives you the option to answer, send direct to voicemail, screen the message being left or answer and record the call. Whoever selects to take the call will get routed the call and the rest of the phones will stop ringing.
If you have GV take a message Google Voice will record the voicemail and then transcribe it. It will then, at your option, email you a transcription of the message. It can also text you a message saying message from XXXX followed by the text of their transcription.
If you choose to take the call you can also transfer it. If you press * the other phones you have set up will start ringing. Someone else can screen and pick up. I use this to go from home to cell and the other way.
We don't have a wired landline and we haven't for many years. With Google's official roll-out they added some features. I purchased a 2-line SIP box to drive the wired phones still in the house. Google will allow a SIP service Gizmo5 to take calls as well. The service is completely free and the quality is excellent. When I pick up on the house phones and take a call the audio quality rivals wired telephone quality.
You can set hours you'll take calls as well. Have different answering messages for different classes of callers in your phone book (such as family, friends, work). You can choose to have different classes of callers sent directly to voicemail and the phone never rings.
The Googlevoice number can also take SMS messages. This is handy for us. My wife has the original iPhone with the 200 text messages. I don't have any text plan at all. So, a text coming in gets re-texted to her but emailed to me.
There's also a one-click Do No Disturb that allows you to silence all incoming calls.
You can also have Google Voice generate an outgoing call and mask it with your GV number so nobody ever gets your cell phone number.
All of this is absolutely free from Google and Google has said it will remain that way with the exception of International calls. Domestic incoming and outgoing calls are all free. International outgoing calls have a very low per-minute rate.
There's also a Google Voice application for the iPhone that allows you to listen to your messages and perform all the calling functions.