I use various apps: Elements, Notes, Evernote (for reading PDF's), Kindle (I don't like iBook's graphics), Calendar, Stack the States and Stack the Countries, Adobe Ideas is great as a whiteboard, Audiobooks is a nice free app though I haven't used it much, and of course Safari. Oh and it is great for YouTube...really nice graphics.
I don't have 3G, but I still love the portability of it. I can read books, take notes, read PDF's, etc.