Yelp launched back in the pre-Mashable era (circa 2004), but after hearing John C. Dvorak evangelising it on the This Week in Tech podcast, I figured I'd take another look. I also dug up this old TechCrunch post which explains how Yelp works:
Yelp is a website that allows users to write and share reviews of local businesses. It also has social networking features (adding friends, groups, etc.) to share reviews with a trust network. The idea is that people generally trust their friends’ recommendations...All of this user-generated content on local businesses, combined with the Yelp search engine, also provides great inventory for Yelp to sell local businesses contextual advertising...There are a number of other companies that are targeting local business reviews in an almost identical way. For instance, check out Judy’s Book, profiled previously and idealabs’ InsiderPages.