Killer application !!!
Developers ! if you can manage to create on of these you will make tons of $$$

1. Plugin for Safari so one can play flash
2. Add the ability to save files to a directory in the iPhone
3. A OS X like ability to manage files \ folders (Finder is nice but the GUI and abilities of the iPhone offer more)
4. Office viewer+editor (like the ones the pocketpc has)
5. Hack to enable landscape mode in all the relevant apps
6. Contextual menus when one holds the finger for 2 sec on the screen
- especially options like Cut\copy\Paste
7. bulk select \ delete
8. Cache for the Browser and Maps \ youtube (offline viewing)
9. Wireless Sync to Pc\Mac
10. Hack the Bluetooth so one can surf the web through it and also sync to a Mac\Pc
11. The PDF viewer is good but one need the ability to store and read PDF not only from emails!
12. Use of gestures like the TabletPc uses
13. View Stocks from around the world not just the NASDAQ