Well Clark Howard likes these shopping apps:
Amazon Price Check - Scan a barcode or snap a picture of an item and then compare against Amazon prices.
BuyVia - Scan a barcode and compare against online prices.
Milo - Lets you search the shelves of local stores at home and on-the-go.
PriceGrabber - Get the lowest prices on computers, electronics, furniture, cameras and more.
RedLaser - Shopping price comparison app. Find the best prices online or locally, and get product reviews.
Shelfbucks - Get in-store personalized deals sent right to your smartphone.
ShopSavvy - Scan a barcode and compare against online prices.
Shopular - Get the best in-store mobile coupons for your favorite stores.
Smoopa - Scan a barcode and compare against online prices.
Walmart's Savings Catcher - Enter your receipt number and if there's a lower advertised price, you get the difference.
You might want to look through this article to see their favorites:
http://www.pcmag.com/slideshow/story/290959/the-10-best-shopping-apps-to-compare-prices
Their choices for shopping apps are:
RedLaser
ShopSavvy
BuyVia
Smoopa
The Find
PriceGrabber
Consumr
ScanLife
ShopAdvisor
Walmart Savings Catcher
The article has a slideshow that shows what each app does.
Also wanted to add that I installed PriceBlink on my computer for online shopping, and have found lower prices at other retailers quite often. For example, if I'm buying something on Amazon for $10 if PriceBlink comes up and says this item costs $5 at XYZ, I'm buying it at XYZ.