Ones I use are
ForeFlight Charts - developed with RVer Dave Parsons of Runwayfinder.com
AOPA Airports
PilotWiz
AeroWeather - fast simple to get METAR and TAF
Weathermeister - not an app but love it. I do not even use the mobile as the full display loads fast and work good on the iPhone. Just create a mobile briefing layout
The Weather Channel - has good annimated radar
Mariner Calc - spreadsheet compatable with Excel and easily down loaded to PC. Use for fuel tracking and log booking.
Pilot FAR and PilotAIM - Nice to have handy but do not need (I mean use) them very often. (wife gave me a iTunes gift card for Christams and needed to pick something)
Air Sharing - Excellent app to put word, pdf, excel and other documents on yout iPhone. I have my POH, Weight and Blance, equipment list, part numbers, plus all the electronic manuals (pdf) for my gps, radio, engine monitor, IO-360 manual. Great to have a lot of these in your pocket when at the airport.