probably the most famous Half-Life 2 mod (other than Entropy Zero 1 & 2)
although not technically a mod anymore, I'm still going to include it still.
It's a Mod / Game where the player can do whatever they want in a Sandbox setting, but its not just sandbox,
they could play TTT (Trouble In Terrorist Town.) Garry's mod has a steam workshop so players can get any mods or addons they want, like gamemodes, maps, reskins, and tools,
probably the most popular Half-Life Mod in 10 years, in the game you play as a metrocop who has to kill some vortigaunts or something in like a bootleg Citadel. the mod is basically Half-Life Opposing Force 2.
Even more popular then the first game, in the game you have been promoted from metrocop to Combine Elite, you have to capture Dr Judith Mossman so the combine can interrogate her, while doing so, you meet new friends and enemys
Probably the best Half-Life 2 mod to ever exist, know for its intense realistic graphics, HD Models that arn't made by gearbox, Great Intense action packed storys with interesting plot twists and awesome weapons,
The Game Starts Of with you in the hall of the Judging Dr Kleiner's, as you stroll on past, you see their leader, after a intense action packed staring contest, you- wait, why should I tell you about this masterpiece when I Could just have you experience it yourself.
Download:
https://www.moddb.com/mods/gordon-freakman
The Mod requires Source SDK 2017, and on the "upcoming mode." I suggest you starting from the intro chapter as it shows you all the High Quality Awsome Action Packed Intense Storyline Game Theory Lore.
Edit: i wrote this in some phase or something idk, the game isnt that good, its not great but its not bad.