To install a Classic Marathon plugin, copy it into your Plugins folder.

Steam can show you the Plugins folder. First, right click on Classic Marathon and choose Properties


Next, choose the Installed Files tab, and click on the Browse button


The data files folder will appear, with the Plugins folder visible.


Copy your Plugin to the Plugins folder, and it will be activated the next time you launch Classic Marathon. To disable plugins, choose Preferences -> Plugins in the game.

• TTEP[github.com]: high resolution (512x512) wall textures for Marathon by Jay Faircloth and Tim Vogel
  
• Updated Starscape[github.com]: A high resolution starscape plugin from Hopper. See README for image credit.
  
• M1A1 HUD[simplici7y.com]: A Marathon-2 style HUD for Classic Marathon by Hopper
  
• Tacticus' M1 Weapons Redux[simplici7y.com]HD replacements for weapons, projectiles, and most items by General Tacticus
  
• Marathon Trilogy Upscaled Weapons, Textures, Monsters, Scenery, Landscapes[simplici7y.com]AI upscales by wrkncacnter

You can find other plugins on http://simplici7y.com