Android: Google Play has a great wishlist function for apps and content for which you're not quite ready to pull the trigger but doesn't have a way of notifying you when content goes on sale or if there is a price drop. Enter AppWatch, a free app that monitors this so you don't have to.
The really nice thing about AppWatch is that you don't have to apps you'd like to follow manually within AppWatch itself, though you can do that if you like. Instead you can click on the share button in the Google Play Store and choose to share to AppWatch.
It's a simple app; there aren't really any frills, but none are really needed. I would like it to have the ability to track prices on other media in the Google Play Store (books, movies, music, etc.) but to be honest I don't really purchase much media through Google Play as I'm invested in the Amazon ecosystem. AppWatch is available for free in the Google Play Store.