Papers, an app from Mekentosj, is available for US$14.99 at the App Store.

Pages
Like it not, we live in the Information Age. As such, we're almost always researching something. Which means we're almost always looking for some place to stash our research where we can find it again. Papers will give you that place on the iPad. Papers is a vertical app targeted at scientific and academic researchers, but it can be valuable to anyone doing any kind of research. It's especially valuable to researchers working across Apple (Nasdaq: AAPL) platforms, since Mekentosj makes versions for not only the iPad, but also for the iPhone, iPod touch and Macs.
You'll also find the app useful if you work extensively with documents in the Adobe (Nasdaq: ADBE) PDF format and like to highlight passages, add notes to pages and share those PDFs with others.