Google Voice gives you a telephone number in just about any area code that you want (though popular area codes may be exhausted), and you can search for available numbers that have strings of certain digits or alphabetic number-equivalents (like 555-LAWS). Plus, you can designate any landline number or one mobile number to ring when the number is called. The technology is all Voice-Over-Internet-Protocol (VOIP), so any call within the United States is free. You also can pay for international calls.
Two features that make Google Voice awesome, though, are its handling of voice mail and its call widgets. Any voice mail that you receive is transcribed and a transcript can be automatically or manually emailed to the address you designate. The audio file also can be downloaded to a computer as an mp3 file. (All of this also works with calls that you record with the built-in recording feature -- perfect for telephonic witness interviews!) There's also a mobile app that lets you review voice mail and transcripts from your cell phone or tablet.
Finally, the call widgets allow you to put a widget on your website that anyone can use to call your Google Voice number. (Check out my contact page to see one in action!) This essentially provides you with a toll-free number for clients or prospective clients, without having to pay the phone company extra -- and it makes you look tech-savvy to boot. Considering that this is a free service from Google (it requires only a Gmail account -- though you get only one Google Voice number per account), it is almost too good to pass up.