What is it?
Crowd sourced opinions and guidance to various services and products, lets you get a better idea of what to be aware of with the product. You might also wish to develop a reputation for reviewing in a particular area and direct traffic to your own blog/ website.
Popular sites
Yelp
Amazon
Glassdoor.com
Epinions.com
Angies list
Cost
Usually free to use and community driven.
Find out more
Is that online review site telling you the truth? - CNBC
12 Places Businesses Should Be Collecting Online Reviews - Hubspot
20 User Review Websites Critical to Small Business - Website magazine