Entirely depends on whether your phone is jailbroken or not As if it's not, none of the apps I'd mention will apply to you

But, AirSharing, from the App Store, is a good app. Basically allows you to simulate "disk mode" like you can on the ipod's, meaning you can transfer files (support for MS Office .doc, .xls, .pps, Apple iWork docs, .pdf, etc), pics, video's, music, - pretty much anything) to your iphone for portability or mobile viewing. Transfer to and from any Mac with a wifi connection - just about any computer that understands either the Bonjour! or WebDAV protocol. Good app IMO

Also there is MiGhtyDocs, which is a manipulation of the cache on the iPhone allowing you to sign in and view any files you have uploaded to GoogleDocs, then cache them to view them offline on the iPhone. Good app, but file support is limited at the mo - spreadsheets don't display well, so I ended up putting all my docs into word files and uploading that way... Still usefull tho, will be better after the creator bothers to update it......

Only other App Store apps I have other than those are either things like ebay and facebook, and games. Most of my other apps come from Cydia and Installer, which you can only access after jailbreaking the phone. Apps like MMS, a text copier app, MobileFinder (like Finder on the Mac, use it to search the root of the iPhone), Winterboard (allows you to customise the look, sounds and settings of the iPhone, etc) Cycorder for video recording, Catergories (create folders within the standard file structure of the iPhone and group apps within these folders on the spring board) and a few others.....