Manta Research's AppSniper is one of those apps that fills a gap that Apple should have plugged at the outset. It's only $0.99 and you'll probably save at least that much the first time you use it.
AppSniper keeps track of all the new, free and on-sale apps currently in the App Store. If you're willing to wait in hopes that a must-have app on your wish list falls to a price you can live with, this app will alert you when it's time to snipe (buy).
You start by scrolling through a list of on-sale or new apps sorted by date. Tap one that interests you and an app info screen will pop up with everything you need to know: title, price, seller, description, and when available, starred rating.
The app info page also displays five icons, starting from the left: display screen caps, app price history, snipe-price setting, ban seller and buy now, which takes you to the App Store. "Ban seller" gives you the option of filtering apps from a specific seller. I don't find that feature at all compelling.
AppSniper features a bunch of config options too: 3G/EDGE on/off, icons on/off, currency, store filter by country, list of banned sellers, and a list of app categories, each of which you can turn on/off.
When I tapped some apps on the lists, the app indicated that the price had changed since it was first listed. The developer notes its database can be a bit behind or out of sync with the App Store and the last word on the price of an app is the App Store's. That's probably obvious, but it's still worth mentioning.
What I would like to see in a future generation is the ability to sort by price, search and a scroll bar or at least up/down arrows to navigate quickly to the beginning and end of lists.
As someone who constantly checks prices, scours the App Store for free apps and buys them by the bucket load, AppSniper will live on my iPhone for a long time to come.
Also check out: BargainBin