AppSync and is compatible with 4.2.1 iOS

AppSync, the application that lets you install applications. Ipa without using iTunes, has been updated to version 4.2 to be compatible with 4.2.1 iOS

To download AppSync we have to Jailbreak your iPhone and add the following repository to Cydia:

http://cydia.hackulo.us

Once you've added the repo in Cydia, the application looking "AppSync for OS 4.2" and installed it.

Source: Redmond Pie