If you use your iPad to read documents or would like to store documents,GoodReader is your app. GoodReader allows you to read PDF's, documents from MS Office, iWork, HTML and Safari webarchives, look at photos and even video. I mainly use it to look at PDF's and word documents. One of the first things I like about GoodReader is how easy it is to get the document into the app. GoodReader allows you to sync up with iDisk, Dropbox, SugarSync,E-Mail, and FTP. I have used Dropbox, E-Mail and FTP and must say it works great.
Once you setup Dropbox you then have access to any file in your Dropbox and simply click a download button to download into GoodReader. E-Mail works much the same way, once it is setup you can pull all your e-mail and see attachments in the e-mail. Simply choose the attachment you want and the file downloads into GoodReader.
GoodReader even allows you to set up folders to store your documents, now I store all kinds of documents that I may need. I do a lot of public speaking and when I get my speech I put in GoodReader and keep it there.
The feature that put this as a must have app is that you can mark up a PDF on your iPad. You can highlight things, cross them out or make notes on the side. The only downside is that I haven't been able to get the PDF out of GoodReader with the markups. It might just be the way I'm doing it but I haven't figured that out yet.
GoodReader has become one if not the most important app on my iPad. It's $4.99 in the app store and for some reason isn't universal, meaning you have to buy it on the iPad and iPhone.