iPhone/iPod touch only: The Dictionary.com iPhone app does just what you might expect it to, but also offers spoken word pronunciations, words of the day, and similarly-spelled options when you're connected to the net.
Based on the Random House unabridged dictionary (and hence likely to offer up US spellings), and boasting more than 275,000 definitions with 80,000 synonyms, the app has just one search bar and four screens. It offers word suggestions as you type, and after picking a word, you can flip between a standard dictionary definition blurb and thesaurus listings. You can check your recent searches and delete unnecessary listings on a helpful Recent pane, and check the Word of the Day on another.
The killer feature here, though—besides the fact that it's free in a field full of sometimes costly competitors—are the audio pronunciations, pulled from the Dictionary.com database and played over your headphones or speaker. Not every word has a spoken version, but the ones that do are pretty clear, and might just save you an embarassing moment before the next time you have to be impressive.
The Dictionary.com app is free to download, though take note of its 35.6MB size—you'll definitely want to be on Wi-Fi for this one. Runs on iPhones and iPod touch models running at least the 2.2 software. Dictionary.com [iTunes Store (direct link) via TechCrunch]