Aimed primarily at developers, but available to anyone, this website lists of all the apps on the Google Play Store. Use the options on the left side of the page to browse by category, country, popularity, ranking, or total downloads. Once you've selected a method of organization, you can click Price Reduced to see currently-discounted programs. The site is handy if you aren't looking for anything in particular, but want to acquire something at a bargain price. For more details, click on any app to see recent drops and increases in price. Unfortunately, AppBrain won't send you alerts, so if you want updates, you need to keep checking the site.