Yelp is one of the largest communities for local business ratings and reviews. Many popular Android applications (like Sherpa) actually pull their restaurant reviews from Yelp. There have been many imitation apps, but Yelp Inc. finally launched their first official Android application.
Version 1.0 is very light on features as you can only search and read reviews. It would be nice to submit photos, reviews, and other content and I expect we will see that in a future version. One nice feature is how easily it integrates with the new Google Maps Navigation. Find any business and in about 2 clicks you can quickly pull up driving directions.
Users in smaller cities might not find much use for Yelp, but if you travel to any large metro area (or live in Austin) it is an awesome tool to find hidden, local places to eat.