Recommended App: AppSales
Here's an app that I discovered recently. AppSales, is a very basic app and the name really says it all. It's an app that shows you which applications are on sale. It pushes notifications which can be customised by the minimum percentage of the sale and how frequently you want to be notified.
It's no secret that the Play Store doesn't sell as well as the iOS App Store and I think apps like this can help convince users to buy more applications. The app also includes sharing capabilities so you can share sales with your friends and family.
Just a small note: The app is still in beta and prices may vary according to currency, location and Tax. Also, I live in an area that doesn't have access to films and music in the market but I don't think the app shows discounts on other than apps.