With recipes so freely available online, it's difficult to recommend paying for a cookbook app. There is one such app I plunked down for, and I feel it's worth every penny.
You may know Mark Bittman from his The Minimalist column in The New York Times. He's also the author of a number of cookbooks, including the excellent "How to Cook Everything." The How To Cook Everything app costs $4.99 for the iPhone or $9.99 for the iPad, but each features the entire contents of the How To Cook Everything hardcover, which costs more than twice as much as the iPad app, even at its discounted price on Amazon. And with its timers and shopping list features, the app does things the book can't.
I have the iPad app and find it fun and useful to browse and search. You browse recipes by category, and you can use the app's excellent search tool, which features five types of filters to help you narrow the results. Each recipe features an ingredients list and step-by-step instructions. You can bookmark your place by tapping on the red circled number of a step, and you can then return right to that step using the Bookmarks button at the bottom of the page. Steps also feature a link whenever prep or cooking time is mentioned, letting you easily set the app's timer for that step. And there is a button in the lower-right corner that keeps your iPad from auto-dimming, handy when you are following a recipe with messy hands.
You can also browse the most popular recipes (you can give a thumbs up to cast your vote on any recipe), or Bittman's Picks, which include the author's top 100 essential, fast, make-ahead, and vegetarian recipes. In addition to giving a thumbs up to a recipe, you can also add it to a favorites list or share it via e-mail, Facebook, and Twitter.
There is also a large How To section, which features clear, black-and-white illustrations of many kitchen techniques. And any techniques related to a particular recipe can be found under the Related Content tab of the recipe. There is also a tab for variations of a recipe and the Notes tab lets you add notes for recipes you may want to tweak when you return to them.
You can add ingredients from a recipe to a shopping list, which you can sort by aisle or alphabetically. Each item on the shopping list shows the recipe for which it's needed, and you can e-mail a shopping list to your roommate or spouse. Simply tap the shopping cart button when viewing a recipe to add its ingredients to your list.
Missing from the vast majority of recipes is any sort of art. Occasionally you'll stumble across a color photo, but for most, you're left to your own imagination as to how a dish ought to look.
If you have a favorite cookbook app that delivers delicious recipes and useful functionality, please share in the comments below. And for another of my recommendations, check out my post on the Cook's Illustrated app.