Yelp (iOS, Android, Windows Phone, BlackBerry; free)
Yelp isn’t exactly a shopping app, but it’s a great resource for user-generated reviews of local businesses, covering just about any type of business you can imagine -- restaurants, retailers, salons, you name it. In particular, you might want to use it to scope out businesses before buying gift certificates or to find a great meal after a long day of holiday shopping. The app also has a deals section, where you can unlock specials at restaurants and stores.
Yelp for Mobile