While Apple will not condone installing third-party applications on an iPhone, we're totally game. The process for doing this has gotten easier and easier, but is still not for the faint-of-heart. And with their recent announcement that there will be official support for third-party Applications in February, it's not too long to wait if you're risk-adverse.
For the impatient, we culled our favorites. Once you follow the instructions to get the Installer app on your phone you can browse that directory to find these fun and useful additions:
Apollo IM A simple, but functional AIM client.
SendPics A utility to email full size images taken with your iPhone camera (they're shrunken to 640×480 by default).
Navizon Not sure where you are? Navizon shows your position on Google Maps by sniffing information from active WiFi networks.
Summerboard This utility lets you install different themes to customize the main menu (Springboard) of the iPhone. Be warned, putting a wallpaper behind your application menu seems to cause lots of crashing.
Labyrinth A virtual version of the the old wood maze and silver marble game, the one uses the tilt sensors in the iPhone so you can control the marble's movement.
Search Why Apple still hasn't added Spotlight to the iPhone is beyond me. This app is a lifesaver when needing to look someone up whose full name you don't know.
Sketches A simple drawing app that let's you draw on a blank screen or over a photo. You can email your drawings just like from the Photos application. The best part is that you shake it like an etch-a-sketch to erase.
VNotes One of the very first iPhone apps—a basic tool for recording, saving and sending voice notes.