Gizmodo How To: Install apps on your iPhone easily, no hacking skills required.

"If you wanted to install third-party native software in your iPhone but you didn't had the necessary technical knowledge or courage, the newly updated iPhone Installer.app [from Nullriver] will make it so easy that it will be very hard to resist. We have tried it in Mac OS X and, as you will see in the tutorial after the jump, it works perfectly —including the installation of applications over Wi-Fi and EDGE."