Cydia is a front-end for Debian APT designed to be a fully-featured replacement for the iPhone Installer, and a mature packaging solution for Unix systems. Cydia works very similarly to Installer, allowing people to quickly get up and running with the newer software. The main interface component is a bar of buttons along the bottom that provide access to the different main screens of the application.
From a user perspective, one should only add http://apptapp.saurik.com/ as a source to Installer and install the package "Cydia Installer" from the "System" category. You might also consider temporarily disabling the auto-lock feature on your device during the installation, as it will take a few minutes to complete. During the installation, you can expect several actions to take place (more information available here - Installation Process).
Download Cydia (Free)
InstallerApp is a native Mac OS X application that allows users to easily download applications from Installer and Cydia directly onto your computer, and install them to an iPhone or iPod touch connected via USB cable. Users just add their favorite repository in the Preferences window and the application will automatically recognize its type, displaying the packages from that repository. Users are then able to download packages to their computer and install them to a USB-connected device.
Also, one doesn't have to access a Wi-Fi network to “let iPhone out” onto the Internet, or download applications over EDGE or 3G networks wasting traffic. With the help of InstallerApp, users download all the required packages straight onto the Internet-connected Mac, and then simply sync all the downloaded packages to the iPhone.
According to developer Ripdev, InstallerApp is commercial software and costs US $7 per license. “By purchasing a license you will add a capability to synchronize up to 3 different iPhones with InstallerApp to install software, get free updates for your InstallerApp 1.x, and also be eligible for the email tech support within a year after purchasing a license,” the developer explains. “Without a license, InstallerApp is a fully functional application, including [the] ability to perform Push procedure and put Installer.app on iPhone — only package installation onto the iPhone is not available.”
Download InstallerApp (Update / Demo / Buy)
Update: article updated to remove download links to Cydia and Installer. Softpedia does not encourage jailbreaking, or using software that requires users to jailbreak their devices.