Cydia by developer Saurik has been around the block a few times, beginning in 2008 as a means of installing and modifying software on jailbroken iDevices. A diverse ecosystem has sprung up around the platform, expanding what iOS fans can do on their usually restricted devices. Saurik's Cydia Substrate, a platform for modifying devices without flashing new ROMs, has now made its way over to Android.
Cydia Substrate does not do anything interesting on its own, but developers can use the platform to distribute extensions that modify software without requiring access to source code. Rooted users are free to load these extensions to pimp out their phones without having to go through all the hassle of installing custom ROMs.