Goodreader (For reading any type of document on your iPad, GoodReader is the way to go. It supports a huge range of formats, and integrates with various online services such as MobileMe, Google Docs, and Dropbox.) (you need an account to see links)
WolframAlpha. You've probably heard of it. It costs $.99, and is a fully functional computational search engine that can solve almost any problem you enter, assuming you enter it right, and give you statistics on almost any subject.