Oh man...I use it for LOTS of things: (in no special order)
-the calendar app. (when viewed in "task mode"...which is so simple yet absolutely engenius--better than the calendar app. on my old Treo)
-I'm always listening to the iPod app.
-browsing the Internet, of course
-calculator, notepad, alarms, timer, and stopwatch--I no longer need to a digital watch.
-The e-mail app. Now I know what BB owners have been gloating about for years.

-the GPS for driving to any place I've never been before. This will have to do til Telenav (hopefully) releases their turn-by-turn app.
-checking the weather
There's probably more but I'm drawing a blank right now,
Brent