Multiverse Mod for FTL

Its literally just a free sequel

Counter Strike and Team Fortress. Both were Half Life mods when they started.

No love for Fallout: London?

It's not just a Fallout 4 mod—it's full-blown AAA experience—done so well that I'd believe it was made by the New Vegas devs if I didn't already know. If you're tired of waiting for Bethesda to make the next FO game, just play this instead.

Surprised no one has said MyHouse.wad yet

Black mesa. Half life and half life 2 modder community is amazing.

Dragon Genitals for Monster Hunter World. Gave each monster either a realistic cloaca or an incredibly well-crafted dong, tailored to the individual style of each monster. The details were amazing, like a monster called Bagel Goose has a rock dick, but when he transforms into B-52 bomber mode he glows red hot with lava… and his cock does, too.

Elden ring seamless coop. I can't believe that such a small file could create such a better experience in the game.

A Link to the Past/Super Metroid combo randomizer. Both games smooshed into a single ROM, with certain doors taking you from one to the other. Item pools are then shuffled together, sending you on a big scavenger hunt to find Metroid items in Hyrule so that you can explore Zebes to find Zelda items so that you can explore Hyrule.

Unfortunately it's somewhat lopsided due to how much bigger ALttP is than SM, but the technical novelty of it is amazing and worth a playthrough.

In terms of actual impact, a Fallout: New Vegas mod named Tale of Two Wastelands that takes a Fallout 3 copy and brings all of it into the Fallout: New Vegas world.

The mod makers didn't create the great bulk of the content, made a converter, but it amounts to a new experience with an AAA-game level of content addition.

Does make me wonder about other mods that can leverage pre-existing work. Cataclysm: Dark Days Ahead had a mod where someone imported a real life map of Massachusetts, for example. Some flight sims can do similar stuff.

Inigo in Skyrim. Fully voiced follower who is funnier, more interesting and more authentic than any companion I’ve had in the last decade of games.

Multiplayer for GTA San Andreas was quite revolutionary at its time.

Withers big naturals

Are Skyrim mods allowed, or is that cheating? 😜

Young me (and current me too) was always blown away by the Game Genie on the NES. Simple piece of tech that could do so much with so many games.

Space Age for Factorio before their basically made it a full DLC for the game

Expanded Stardew Valley! So much added stuff!

Space Exploration for Factorio. My dude built a huge mod for free, then was hired by the devs to desogn the paid sequel, Factorio 2, and although large and great its still smaller and less complex then the mod.

There are quite a few honestly. Team Fortress and DayZ are obvious examples, I also quite liked Natural Selection.

For me personally it has to be certain rom hacks from before the era of decomps and creation kit style tools.

I was fairly active in the gen 3 Pokemon hacking scene on the pokecommunity forums back in the day. And hex editing and trial and error was one thing but real wizards were the ASM hackers that could completely out of the box stuff like adding in new move effects or character customisation functionality or day and night systems, to games that had none of these.

Those were actually the trailblazers that led to all the decomps we have today that have made it so easy.

Vampire: The Masquerade - Bloodlines Unofficial Patch by Wesp5.

That game was already broken at launch, and has become more broken as operating systems advance. Furthermore, Activision shuttered Troika almost immediately after VTMB was released, so there has really never been any official support. But, Wesp5 has been working on the Unofficial Patch since shortly after the game came out in 2004.

It fixes the game to a completely playable state, which is a miracle for VTMB. He basically just finished the game for them.

Furthermore, there was some controversy over some of the changes that the patch started implementing, so he split the patch into two versions. You can download the True Patch, which fixes the game to a playable version of its release state, but Wesp5 also offers the Plus Patch, which restores all content cut from the game before release. He reimplemented character backgrounds, real-world weapon names, multiplayer, quests, locations, music, and the list goes on.

It's arguable if the Plus Patch actually represents the original vision of the creators, since we don't know why all of the content was cut. A good bit was cut for time, but there were some things that were dropped earlier in development. Either way, the Plus Patch went feature-complete last year, so what we have now is a fully working version of the game containing absolutely everything that was ever planned during its development.

I don't know if anything like that has happened before, and the only other thing like this that I can think of is the Saints Row 2 patch that IdolNinja was making before he died.

Mods that add stuff are great, but it's hard to beat a mod that lets you play one of the greats.

Fall from Heaven 2 (overhaul mod for Civ4) would've been top pick until this year, now it's Enderal (even calling it an overhaul is selling it short, it's a whole new game built on the Skyrim engine).

Pokemon unbound. Its a completely new game.

Does the flight sim in Excel count? If yes, then that.

Otherwise, Day of Defeat (Steam mod). Still somewhat peeved that it never got the same recognition as CS.

Super Mario 64 that added lugi as co-op

Doom: Infinite is pretty cool. It turns it into a roguelike. It’s amazing as is, and is still in beta.

Cursed Halo.

I believe it was a passion project by one guy that took 5 or 10 years but holy shit it's a masterpiece.

Strawberry jam for celeste. It's a huge collaborative project that introduces 100+ levels (base game has 25) with a really wide range of difficulties, graphical styles, new mechanics, original music and ideas on par with the quality from the base game. Definitely a must play if you like 2d platformers

Cataclysm: Darker Days Ahead.

Daily pull requests for over two decades on an open source project

Star Trek New Horizons for Stellaris. Best Star Trek game I've played.

The entire OAR for Fallout 4:

Outcasts and Remnants

Fusion City Rising

Depravity

Project Valkyrie

Save Our Ship 2 for RimWorld basically turns the base game into a sequel of itself.

Tamriel Rebuilt for Morrowind is also pretty dope and it isn't even finished yet.

DSfix

Single-handedly made an awful PC port playable.

Since someone already mentioned Fallout London, then I'll recommend Shutoko Revival Project for Asseto Corsa, a full blown open map in the speedways of japan with online support and even functioning traffic so the illegal racing isn't a breeze.

X-Com and X-Com 2 Long War mods. Essentially the games at their full potential.

That reminds me, It's almost time for another playthrough i think.

Princes of Darkness for Crusader Kings 3

Midian XL for Diablo 2

Doom 3 OpenCoop. There's a few mods that convert the single player campaign into online co-op. At the time the competition was Last Man Standing, but I liked how vanilla OpenCoop was. There's better mods now, and OpenCoop never left Alpha versions. Although the only game breaking bug is with the first teleporter sequence which kicked all clients from the server. Easy enough to skip that segment.

It's a bit of a scripting nightmare since the game uses so many floor triggers for events never intended for multiple players. That's what impresses me about the mods.

That or The Dark Mod which is now a Standalone freeware title using idTech 4 engine to make a fan sequel to the classic Looking Glass Thief games. There's a TON of fan made high effort maps to play on.

Team Fortress and Team Fortress Classic.

Witchery for Minecraft. Start out, building your altar room, find as many unique living things as possible and put them within 100 blocks of your altar. Mid game, make sleeping tonics to visit the spirit realm and try not to die from nightmares. Create voodoo dolls, and other nice high level gear, craft some powerful pvp oriented spells. Late game create a ritual room to summon demons to trade with. Don't forget your protection circles because they move really fast and kill you almost instantly. Late game, become a vampire or werewolf or hybrid. Become nearly invincible outside of fire and sunlight.

Carball for Unreal Tournament 2004.

They completely changed the game to be a proto rocket league game many years before rocket league was a thing.

There are a TON of legit and amazing S.T.A.L.K.E.R mods like Misery, but my favorite is Anomaly. The mod team took assets across the three original Stalker releases, took alpha/beta builds, and combined all of these with a ton of other mods to make an ultimate OG stalker trilogy playground that is open and full of immersive experiences. Almost to the point where you can randomize starting characters and basically never play the same game again with how the NPCs have full independent routines.

Of course this means that a random quest giver can totally just get zeroed out and die before you ever even meet them, but that's part of the experience lol.

I came across the Star Wars Helldivers mod. It’s basically Battlefront 3 but man it looks cool

Command & conquer Generals Zero hour, Shockwave.

It's lesser-known, but the Battletech Advanced 3062 mod for Battletech from Harebrained Schemes is incredible. It adds so much it's almost an entirely new game (but still Battletech).