Scrivener and Goodreader. The latter stores a lot of PDFs and such for me. The former works out pretty well for working on documents while I'm out and about (versus taking my laptop). A nice feature of Scrivener is that you can attach files as well. So it's turned out to be a great study tool (that split-screen apps kind of eliminate the need for) because I can open a PDF of a text and have it adjacent to my notes on that section or the part of the paper I'm writing that's relevant to the PDF.

My secondary use case is board games. Carcassonne and others have pretty good ports which was very handy for playing with people while traveling or instead of carting a bunch of boxes (or bags) over to friends' homes. Everyone but me has gotten married and had kids so gaming is much less frequent these days, and usually hosted by the guy who has the biggest board game collection when we do gather.

My final (present) use case is tabletop roleplaying games, but that's also covered by my current device. Hero Lab has an iPad port which was handy for some games I ran in the past, and hope to run again in the future.