We previously reported on the Installer.app single-command installer, which makes it possible to put an application on the iPhone that in turn installs other third-party applications directly from the device with a single script launched by the Mac OS X terminal.
There's now an actual Mac OS X application with a GUI that performs the same function, meaning interaction with the Terminal is no longer necessary to get third-party applications installed and running on the iPhone. Dubbed "AppTappInstaller," the program allows the user to choose which version of the iPhone firmware they are running (1.0. 1.0.1 or 1.0.2) then performs a series of functions that automatically jailbreak the iPhone and push Installer.app to the device.
For more information on using Installer.app once it's on your iPhone, see this article.