In the past I've used the Yelp app to locate the nearest good restaurant after arriving at an unexpectedly closed one with a party, I've used the check-in feature to place my virtual footprint on the hallowed pages of the places I visit, I've occasionally used the monocle feature to see through walls to about where the next restaurant in my night is, and I've even taken advantage of check-in offers.
Integration with Google Maps, having a community-backed quite-oft edited database, and reviews you can usually trust have been a huge plus, but now there's more.
Yelp has decided to take on Groupon in the smartphone selling coupons game. Groupon has pretty much cornered the market of half-price coupons, which generally net the business you're using the coupon at 25% of what they normally charge the customer, with the other 25% of the normal price going to Groupon.
Yelp is undercutting Groupon's price to businesses in an attempt to garner more of the market share. What that should mean to you is more of the restaurants you love will start selling discounts, as more of the money you're paying is getting to them.
The Yelp app is probably not going to change your life, but you may come home with a few more dollars each week, and you might find some friends on there. Yelp Deals are in limited markets, so check to see if you're included.