Xposed is a framework on Android in which modules (in form of apps) can take advantage and do magic. It's so powerful that if the framework or modules are improperly configured you may end up screwing your phone.
Search for xposed tuesday on YouTube and head over to xda to install xposed.
Gravity box - has tons of tweaks to improve roms.
YouTube adaway - no more ads on YouTube.
Niwatori - replicates apple's double tap to shrink screen size.
Xprivacy - privacy manager.
Always have the uninstaller ready for safety measure.