One of the most useful apps for doing a site like this is AppShopper. AppShopper is great for discovering new apps. There are few things this app does, first it gives you a list of all the apps that have changed and it's a continually updated list. The updates could be a new version, price change or totally new app to the app store. There are filters that you can select so that you see only apps from certain categories, that have been updated, new, price changes, or that are free.
One of my favorite features of this app is the wish list. If there's an app that you want, add it to the wish list and every time it gets updated you receive an alert. This is great for watching an app for a price drop. Most app (expectably higher priced apps) usually go on sale or drop in price. I've picked up several $6.99 apps for $.99 or even free.
I would say about 80% of the apps that I review on this site I found through AppShopper. This app is free and a must have.