Not necessarily 3rd party apps, but enhancements that I'd like to see to the iPhone.
Notes
* Categories for notes.
* Synchronization of the notes on the iPhone with something on the desktop.
* Ability to search.

Calendar
* Categories for calendar entries.
* Monthly view with entries indicated by color per category (so I can tell if my plans for three weekend's from now are HAM related or Personal or Motorcycle or Work, etc. at a glance).
* More flexibility in the alert times (I personally like 10 minute alerts and that's not an option on the iPhone).
* Better options in adding a repeating event; I'd like to have something that repeats monthly on the 3rd Monday for example.
* Support for icf files attached in e-mails to accept/upate calendar entries.

Photos
* Ability to put pictures taken on the iPhone in different categories/folders
* Ability to create new categories/folders

Clock
* Big screen clock with an option to turn off the auto lock from this screen (while this screen is open).

Calculator
* Something more than a very basic calculator. I really liked the calculator on my Treo which allowed (among other things) unit conversions (I use that all of the time).

Mail
* Set Auto-Check on a per account basis.
* Set the "From" address to something different than the account settings.

Google Maps
* Ability to select an area in Google Maps and say map to/from this point without needing to search for the area by name/address.
* Support for a bluetooth GPS.
* Ability to link from calendar entry "location" to Google Maps.

iPod
* Ability to fetch podcasts from the iPhone (without connecting/syncing).
* Ability to delete stuff (music, podcasts, etc.) from the iPhone.
* Ability to mark podcasts as items to not delete from the iPhone.

Phone/Contacts
* Categories for contacts.
* Ability to search/sort by something other than name.
* Support for vcf files attached in e-mails. Should allow import/update to contacts.

Other
* Synchronization with Lotus Notes (Calendar, Contacts, Notes).
* Voice recorder.
* Ability to move the icons on the "home" page.
* Ability to add applications to the "home" page.
* Ability to add applications to the iPhone that do not require network connectivity to function.
* Ability to search across all data in the iPhone (i.e., calendar, notes, contacts, e-mail).

http://mckeehan.homeip.net/wiki/wikka.php?wakka=iPhoneWants

William