"Don't forget that Google Maps indexes a tremendous variety of businesses, and in a number of countries. Tell the world all about your favorite (or least favorite) doctors and dentists, hotels, bakeries, hardware stores, salons, pet spas, auto mechanics, plumbers, and more," recommend Google Lat Long Blog.
Just find a local business, click on "more info" and then find a link that says "write a review". Your review must be at least 100 characters, it will go live instantly, but it's placed after the bottom of the page.
