Our sister site's AppShopper App has recently seen a few updates, including the much requested addition of Push Notifications.
AppShopper tracks all the major changes to the App store, including new apps, updates and sales. For example, with the right combination of filters you can find every app that has gone from Paid to Free. For a less overwhelming view, you can just view popular changes amongst other AppShopper readers.
It's what we use to keep track of interesting game sales, and now you can monitor them on your iPhone or iPad.