Addition released their application iPhoneModem Saturday through the MMi Cydia source. They describe it as "an application for the iPhone that aims at making it really easy to use the iPhone's internet connection on your computer," and they deliver.
A while back, we posted a how to on tethering the iPhone using NateTrue's method - which required some command line knowledge and a few network tweaks on your OS X box. Until now, this was pretty much the easiest way to tether your 3G iPhone (and still is a great method which only takes a few minutes).
iPhoneModem makes tethering a 10 second process. I've been testing it the past few hours, and its incredibly well-designed and functional. For those of you running OS X Leopard (iPhoneModem is actively working on the desktop client for MacOS 10.4 and Microsoft Windows, as well as a 1.x iPhone..