Well I don't have an iPhone yet, but this is what I intend to install.
Installer.app
BSD Subsystem - Needed for apps to work
Open SSH - SFTP access to the iPhone files
SummerBoard - Extend SpringBoard
rSBT - Rearrange Springboard from iPhone
MobileFinder - File Browser
MobilePreview - Preview Images and PDF's
MobileTextEdit - Edit text documents
Lights Off - Lights out game
VoiceNotes - Voice Recorder
iShare - Upload/Download from sendspace.com
What do you think? I don't want to clutter the iPhone with pointless apps, but would like to add extra functions like file management and nice games like lights out!