FreeAppAlert is a website that does exactly what it says it does- it provides alerts of paid apps that have become free. It provides a link to iTunes and occasionally you’re bound to find a gem. It normally posts daily on its Twitter feed featured apps, which may be better rather than searching daily for a bargain!
Examples of apps that have been temporarily free for promotions includes Driver (GameLoft Christmas promo), selected Lonely Planet apps (Stuck in a European city? promo), Ragdoll Blaster and Chop Chop Soccer (Free App a Day promo).