I just upgraded to an Android-based phone, so I've got the same questions... what don't I know about?
I don't know if they're available on iPhone or not, but categories of things that have been suggested to me include...
- Anti-virus (Lookout is what I have)
- Process clean-up (Task Killer... allows me to close things that like to stay open after I leave)
- Bar code scanner (can use to scan and compare prices while shopping)
- Flashlight (included with Android)
- Angry birds (game... my kids love it, and I enjoy it too
)
After that, it depends on what you might want to do.