The most popular tweak in Cydia, AppSync is now available for iOS 4.2.1. It lets you install cracked apps on iOS 4.2.1 firmware by patching the MobileInstallation file on iOS 4.2.1. Once you have installed AppSync iOS 4.2 on your iDevice running jailbroken iOS 4.2.1, you’ll be able to install cracked apps (.IPA) on your iDevice without using iTunes.