Here's another service that greatly benefits from being an app, instead of a (mostly) ad-cluttered website with information overflow. The Kayak app is simple and does what it's there to do: Find the best hotels. You'll get reviews, directions (using Bing Maps, what else) and photos for most hotels in the world.
What? No YouTube'? Not a problem: Vimeo offers mostly higher-quality videos and is not nearly as littered with stupid clips. The vimeo app is probably one of the most beautiful apps out there and sports a very -- dare I repeat it -- "fast and fluid" experience. And yeah, it's really fast and fluid.
Use the Dictionary.com app for pretty much anything you'd use a dictionary for -- except maybe an impromptu booster seat or DIY flower press.