iPad Apps, iPhone Apps, Deals and Discovery at App Shopper - Popular Recent Free Price Changes in Games for iOS
appshopper is also available as app you can install it, and add items to the wishlist, so you get an alert if something you want drops in price
before buying anything, check appshopper for history to see if there were prior price drops. if it has gone free or price dropped before, it is likely to hit that price point again.
also touch arcade posts mosts of the good price drops and freebs, plus their forums has a promo code give away section, but these codes are used up soon after they are posted, and are normally used for promoting new games
Dark Nebula - Episode One is free