You could argue that there's not much need for a separate dictionary app any more. With dictionary services built into nearly every interface and Google and Wikipedia searches just a tap away, it's not hard to find out what a word means or how it's spelled any more. But sometimes you may just want to browse around or look a word up for yourself, and for those times, there's the Merriam-Webster Dictionary app, which is now available on the iPad for free.
It's certainly not the only dictionary app available, and the renowned Oxford English Dictionary has its own edition available for the iPad already. But that one costs US$54.99, and the ad-supported Webster dictionary benefits not only from a lack of initial cost, but features like voice search, audio pronunciations, a search history and a browse mode that lets you search and learn new words as you like. It's a solid dictionary app from an old name in the word-indexing game, and even if you only use it occasionally, it's probably worth having around on your iPad.