Yelp

Yelp sells ads to local businesses in city editions within the USA, Canada, United Kingdom, Ireland, France, Germany, Austria, and The Netherlands. As a paying advertiser, you cannot change or re-order your reviews. To protect businesses, Yelp has an automated filtering system that suppresses the small portion of suspicious reviews.

More information about them, from the website:

With over 63 million unique monthly visitors to the site, a rapidly expanding user community, and a built-in forum for consumer interactions, Yelp is a fantastic place for advertisers to reach an actively engaged audience.

Yelp is also available for cell phone/mobile advertising with:
- Yelp for iPhone
- Yelp for BlackBerry
- Yelp for Palm Pre
- Yelp for Android

You need to visit their site to determine if the metropolitan area you target happens to be covered. They seem to be very active and are expanding quickly.

Visit Yelp