A pretty obvious reference facility for any device is a dictionary, and Dictionary.com's free app is highly recommended. It's extremely comprehensive (over 500,000 words), with a pronunication guide (for which you need to be online), reliable definitions from the Random House Dictionary, etymology, encyclopedia references, a thesaurus and a recent searches list. It won the "2009 Best App Ever Award for Primary and Middle School Kids." All very impressive, and for no outlay!
Download it here from iTunes (free).