Developing for jailbroken iOS

This is a collection of knowledge about jailbroken iOS development, written by and for developers. It includes tips for getting started, a list of open source extensions, and documentation for many classes in private frameworks. Run by DHowett.

This is saurik's documentation for Cydia Substrate (sometimes called Mobile Substrate on iOS), the code modification platform that is underneath most extensions in Cydia.

This wiki, started by geohot, does not have much detail about extension development, but it does have a lot of technical information about iOS itself, iOS devices, and jailbreaks.

If you're interested in learning about how jailbreaks work, this is a reasonable place to do some reading.