Before I get into specifics, here's a quick overview of what Google Voice does. Note that Google is rumored to be merging Voice's features with the more popular Google Hangouts later this year. At that time this process could change.
When you sign up - click here for the Google Voice page - Google gives you an account with a new phone number, or you can transfer an existing number for a one-time fee. After the account is created, you can add your existing phone to the account. Once that's done, you can give your Google Voice number to friends, family, clients and business, and Google acts like a personal assistant checking voicemail, routing calls and much more.
Google Voice has a few different feature levels. You can sign up for a Google Number account which gives you a new number and every feature; Number Porting lets you use an existing phone number and every feature; and Google Voice Lite gives you a new number but is just for voicemail and International calling. There are also two exclusive options for Sprint customers that tie more closely into your phone.
I'm going to be looking at all the features Voice has to offer. If you don't see some of these, you might be signed up for Google Voice Lite. In that case, go to your account, click the Gear icon in the right corner and choose Settings. Then on the Phones tab next to your number, click the "Get a Google Voice number" link and follow the directions.
Note: There aren't a lot of free phone numbers available for some areas. Phoenix and its surrounding cities didn't have any free numbers when I checked, for example. So you might have to look in smaller cities or other parts of the country.
So let's go in-depth with three of the coolest features you get with Google Voice, and I'll quickly tell you a bunch of other features you might like.