Installer.app lets you install applications onto your iPhone using WiFi or EDGE. You can also use it to update or uninstall said apps. Which is handy. Harry. Not official, no support, use at your own risk.
New! We now have a single command Mac (PPC & Intel) installer for Installer.app, get it here. This will automate the entire jailbreak and Installer.app installation process for your 1.0 or 1.0.1 iPhone, including automatically downloading the right firmware files. Be sure to read the short README file for installation instructions. If possible, it is recommended you do a full restore of your iPhone using iTunes before installing this, but this is not required — it will simply make for a cleaner install.