I have a 3G (not even a "S") and find it to be useful.
Calendar: Stock AP; you can auto sync with Google Calendars and share calendars.
Mail: Stock AP; You can get your POP3 mail right to your phone.
iHandy Level ; Works OK, but calibrate it to something you know is level before checking level on something you don't
MotionX makes 2 products: GPS Drive and MotionX-GPS. GPS is Drive is just like the GPS standalone devices you see folks with on their dash (I don't have one, I just use GPS Drive instead). MotionX-GPS works like a handheld hunting GPS, but updates maps as long as you have atleast E service. You can also download maps for areas you are interested in. It will do "Tracks", and you can mail yourself a copy of the track overlayed with a Topo Map for no extra expense.
SoundMeter and dB sound meter: Sound meters. I use them to verify equipment I install meets minimum SPL standards.
DipSwitch: If you are weak at Base 2 adding, this app will assist you to set dip switches on Base 2 addressed devices.
There are quite a few more. You think of it, and someone has probably made an APP for it. I do my business expenses via an APP, can Bank via an APP. Do spreadsheets with Office Plus. Read up on Fly Fishing with the American Fly Fishing Mag APP, etc.
There is a real neat one for the 3GS and up that helps you identify constellations in the sky. You just point the phone in the are of the sky you are interested in, and it shows you the name of the constellation you are looking at. Only works with phones with built in compasses, so it won't run my poor old 3G, but is fantastic on my GFs 4G.
Moab: That is a neat little APP
