From iPhone Development Wiki

Hacks for testing things on the simulator without compromising our devices with a beta release.

angelXwind/simject is a command-line tool and iOS dynamic library that allows developers to easily test their tweaks on the iOS Simulator. It automates the process of manually loading substrate and additional binaries in the simulator runtime.

For projects older than iOS 7, the following guides detail the hacks used to load libraries manually in the Simulator: