The OS Mockup Kit is a large collection of assets used to create OS mockups, which are basically mockups of fictional operating systems. Originally based on feandesign's WNR (Windows Never Released) kit, it has evolved into eight much larger packs, thanks to Glosswired who took over the project at the tail end of 2017. Currently at version 1.2, it contains both official and fanmade assets, although there are also the OS Graphic and Sound kits, which lack most of the fanmade assets (only exception being the unofficial Longhorn and Whistler sounds, due to their popularity).
It consists of 4 parts:
There are other variations of the OS Mockup Kit, including:
Even if you aren't going to use it for mockups, you can still go ahead and download it if you're interested! It can be used for pretty much anything, from weird Photoshop experiments to making websites to modding video games.