National Geographic Society produces some of the most compelling, fact-checked content money can buy and their World Atlas iPad app has long been my go-to reference source, not just for checking out interesting facts and trivia but for doing serious research related to the world's countries, their economies, demographics and so forth.
They updated the app on January 11 with high-resolution Retina graphics, revamped UI and added a new feature which lets you explore a full 3D view of the globe.
Today, they're making the app free for a limited time and you're advised to download it now before the price goes up. Note that this is the first time ever the app has become available as a free download since its 2010 App Store debut...