In order of decreasing usefulness:
My personal killer ap on my Visor is Noah, a dictionary program. Ideally, I would tuck my unabridged dictionary into my hip pocket and take it with me everywhere. Noah’s the next best thing. This is why I’m glad I have 8 Megs in my Deluxe, so I can have the largest database that Noah offers.
I like Handyshop for grocery lists. I also use it for longer-term list-keeping, like my list of “Books to Read,” “Books and CDs to Buy,” “Books Loaned Out,” etc.
The calculator is very useful. I’ve programmed in many unit conversions that are helpful for work and grading. My Visor is actually smaller than my scientific calculator, and, except for graphing, it can do pretty much everything I need to do on a regular basis. You can download better calculators (included graphing calculators), too, but I just use the one it shipped with.
I have a doc reader. I always have couple of Guttenburg ebooks, plus tons of miscellaneous gaming notes. A keyboard is pretty much a must if you want to do any serious text entry.
I have several games that I play pretty frequently, when I get stuck in a long line or waiting at a bus stop. Mostly I play Patience (an excellent freeware solitaire program) and PictureLogic and Pikoban.
I use it for phone numbers and addresses, too, though it’s not clear that it’s a significant improvement over carrying small address book. A friend of mine had a huge database of phone numbers for restaurants that deliver, and she beamed it to me, and that was pretty cool.
I rarely use the calendar, because my life is really not so complicated as to necessitate one.
I had a whole bunch of other stuff on it, too, but a month ago I had an unscheduled hard reset (I neglected to replace the batteries in a timely manner) and I didn’t bother to reinstall half my apps. Every once in a while I think, hey, I could use this or that . . . and then I remember that I didn’t reinstall it, and I usually just shrug and think, oh, well.