The new Dictionary.com app, released this past week, is free and includes a dictionary with more than 275,000 definitions and thesaurus with 80,000 synonyms. Note that this is not simply access to the online site; rather, you get a full dictionary on your device that you can use offline. Of course that means that it takes up a chunk of memory. The App Store says the app is 35MB, but my free memory seemed to be reduced by about 240MB after I downloaded and installed. Other great features include audio pronunciations (for which you'll need an Internet connection), similarly spelled words, and Word of the Day.