It is a
website that lets you track iOS and OS X apps - including price changes, news, updates, etc. - and can notify user via email if they desire. It does a lot more than the App Store. The
AppShopper Social app lets you share your app interests with your friends, see what apps your friends are interested in, as well as keeping you up to date on the newest App Store apps, sales and freebies. The website and app are both free.