On a related space theme, my family weren't sure about my iPad until I showed them Star Walk for iPad and then I had trouble getting it back from them! It is a lot of fun even just playing around in the daytime.
I have also been loving the GoodReader with a Dropbox account (although I had that with iPod Touch as well, and I'm a grad student that has to read 100+ pages of journal articles a week, but it is 1000 times better reading them in an iPad than iPod). I forget if GoodReader is free or cheap, however. I think the Dropbox app is free and works almost as well.