Flex is a paid tweak in Cydia if you are jailbroken. It has a community of folks who create tweaks and upload to the Flex Cloud. You can browse these tweaks and apply them to apps you have installed from AppStore and stock tweaks. Or you can manually create your own and upload them to the flex cloud.
Things like add time stamps, photos to iMessage or remove sponsored ads from Facebook etc. quite a cool little app.