I see this was mentioned in the Android deals thread but I have a feeling that most people rarely check that thread and would probably miss this.
From 12/24 to 12/28 if you download any app from the Amazon App Store on your phone/tablet they're going to give you a free $5 credit for the App Store. That includes the free App of the Day. You do have to be running the latest version and it doesn't count purchases from a Kindle Fire.
In addition to this limited time offer, all Amazon Appstore customers will find a paid app for free every day, including Angry Birds Star Wars II and Where's My Perry?, as well as exclusive Free App of the Day bundles on December 25th and December 26th. Other top titles will be discounted by 50-90%
-- December 25 - Free App of the Day Welcome Pack, which includes My Alarm Clock, Handrite Notes Notepad Pro, Stellarium Mobile Sky Map, Relax Melodies Premium: A White Noise Ambience For Sleep, Meditation & Yoga, SPC Music Sketchpad 2, PrinterShare Mobile Print, and Moon Phase Pro
-- December 26th Office Suite Pro 7 - only $0.99 (originally $14.99)
-- December 26 - Free App of the Day Favorites, which includes Daily Ab Workout, The Lost City, Voxel Invaders, Chimpact, Osmos HD, Fraction Calculator Plus, Ultimate Hangman HD, From Cheese, and Business Calendar
-- Free App of the Day deals:
December 23 - Draw a Stickman: EPIC
December 24 - Piano Master Christmas Special
December 25 - Angry Birds Star Wars II
December 26 - Where's my Perry?
December 27 - Quell Memento
December 28 - Doodle Farm