This is called Google Tags and it's not done through Adwords it's done through Google Places (previously LBC).
It only costs $25 a month. But be aware it won't help you rank any higher so if you're buried on page 3 of maps it won't help much. However if you have a 1st page listing in the 7 pack it can really help your listing stand out, as it does in the example you shared.
Here's the Google announcement from when they expanded it to more cities. It covers the cities where Tags are currently available and some of the uses. It's not just for coupons.
We're adding a Tag for one of my Chiropractor clients who we just got to #1 in a large metro area. I'll be watching his stats closely, so we can see if the Tag gives him a boost in actions.
But for $25 a month, I don't see how you can go wrong.