so far...

my iPhone is still 1.0.2

I use/Play most:

Installer : Number 1 app. A must definitely. You can't install anything w/o it.
iSolitaire : Famous PC game. When you open all the cards A's fall down and move as you move your iPhone.
iGo : A very clever Go game. I could not win a game yet.
Safari : I check my mails, read newspapers.
Categories: You can sort your icons into folders. Useful for the ones who installs many apps.
SummerBoard : Extends your springboard (home screen) to have more icons. A must for the ones who installs 3rd party apps/games. Without it you can't find apps you installed and you can't run'em.
Customize : You can change the order of your icons. Not as amusing as in 1.1.3 but useful.