I've been using GoodReader since the iPad first shipped and didn't realize its true value until a conversation I was having yesterday. If you're not familiar with GoodReader, it's an app that lets you view a wide assortment of file types on your iPad. Included in that list is support for large PDFs and TXT files, MS Office docs, iWork, HTML, images, audio, video, and more.
While the support listed above is cool enough, the feature that makes GoodReader a must-have on my iPad is its support for Apple's Document Sharing. The support allows you to not only launch and view the document on your iPad from compatible applications (such as Apple's Mail client) but also store it inside of GoodReader. So, no more hunting for that doc in your Mail the next time you fire up your iPad. Instead, just launch GoodReader and your previously launched docs are sitting safely in GoodReader's interface.
I mentioned earlier that I didn't realize GoodReader's true value until a conversation I was having. Well, it almost felt like an infomercial, since a friend was saying that he loves his iPad but wishes there was a way he could store documents on it and view them without having to load them up in Mail each time. I heard myself saying, "Now you can!" and the rest is history.
There are so many features loaded into GoodReader that I can't begin to cover them all. For example, you can launch the docs from compatible apps or sync docs to the iPad via Wi-Fi.
GoodReader for iPad is currently selling for $0.99. For that price, if you need to view large files or are concerned about full support for viewing a particular doc on your iPad, it's a small price to pay.