(click on the title above to go to the website)
From the website: "Yelp is the fun and easy way to find, review and talk about what's great - and not so great - in your neighborhood and beyond. You already know that asking friends is the best way to find restaurants, dentists, hairstylists, and anything local. Now, Yelp makes it fast and easy to make and receive real reviews from real people." - It covers the SF Bay Area best, but is expanding to other US cities. Hopefully the rest of the world will come next.
Note that CitySearch.com has many more reviews for more places than Yelp, including its own Editorial Profiles. However, it has more of a Web 1.0 interface (feels like the Yellow Pages) and lacks social networking and community building tools.