This is a good little app for tracking price changes for apps on the iOS store. It works in a similar way to camelcamelcamel for Amazon, in that it tracks and stores the price changes of virtually all the apps in the store. Therefore if you want to buy an existing app, you can see whether it has ever been cheaper historically, and then decide whether to wait.
You can also set-up alerts for apps you want, and the app will send you an email when it reaches the price you're prepared to pay!
Finally, it's a great way to find games or other apps that don't necessarily reach the high points in the charts on the store, and therefore are harder to find in the first place! I've found several great titles through it myself, so hope it can help some of you!