Before the easy availability of free-to-use game engines like Unreal Engine, would-be game developers had a problem. How do you build an entire game from scratch when you're not John Carmack? Well, you make a mod for an existing game, like the DOOM and Quake engines that Carmack was instrumental in creating. Plenty of these mods were left by the wayside, but a select few turned into commercial successes. And the best part? Because most of them are older, you don't need a super-powered graphics card to play them.

So, in no particular order, here are some of the best game mods that turned into commercial successes.

12 Insurgency

Started out as a Half-Life 2 mod before becoming a full Source Engine title

So many game modders use Valve's Source engine as a starting point for their games, and Insurgency is just the first of many on this list. Unlike the more well-known Source mods, this game was brutally hardcore, going more mil-sim than most FPS titles. You don't get a virtual crosshair to aid in aiming, or much in the way of a HUD, making it far more chaotic than many multiplayer titles. It was a refreshing change from the normal multiplayer setup, and the original mod became a full release; then it got a spin-off and a sequel, Insurgency: Sandstorm.

11 DayZ

This post-apocalyptic zombie survivor title started out as a humble ARMA mod

Ahh, the days when everything was a zombie game. It was a simpler time, but DayZ's combination of survival horror and hastily made alliances with other players is a winning formula. It started life as an ARMA 2 mod, dumping the game's characters into a zombie-infested world, but not a whole lot changed between the mod and the full game release. That's a good thing because the game is fantastic and well worth checking out if you like survival games.

10 Dota 2

The most-played game on Steam started out from more humble beginnings

Even though DotA (Defense of the Ancients) wasn't exactly the first MOBA on the block, it's certainly the one that stuck around the longest, cementing the genre in esports history. Originally made from a modded Warcraft III map, DotA changed up the RTS formula, shrinking the number of player-controlled characters to one and having them fight through mobs to level up before fighting each other and taking out the opposing base. Then DotA Allstars came out, got its own tournament thanks to Blizzard, and the rest is history. When two main modders of DotA Allstars went their separate ways, one went to Riot Games to develop League of Legends, while the other joined Valve to create Dota 2.

9 Team Fortress

Originally a Quake mod, this classic got snatched up by Valve

Back in my late teens, there was only one game I would play for hours on end while connected to the internet on a v56 modem that regularly gave me three-digit ping latency. That game was Team Fortress, which started out as a Quake mod, and it had me hooked. The original hero shooter was unlike any other multiplayer game of the era. Instead of the deathmatch arena shooters it was based on, it let you choose which class with a specific set of weapons and skills to use.

It was so popular that I remember more people being on the TF servers than on the normal Quake ones, and one company took note. Valve hired the modders responsible to create Team Fortress Classic, a rework inside the Half-Life engine. Nearly a decade later, Team Fortress 2 was released, and it's still one of the most popular games on Steam today.

8 The Stanley Parable

Seriously, is there nothing that can't be created in the Source Engine?

If you haven't already played The Stanley Parable, I only have one question. Why not? This fantastic story-driven experience is all about choice, so I guess you don't have to do what I asked, but you'll be missing out on one of the best gaming experiences around. Originally designed as a mod for Half-Life 2, this seminal surge of storytelling is a bit like having your own personal narrator, if that narrator sometimes chips in to offer suggestions about what to do next. Make some choices, and yes, doing nothing is also a viable choice, and you'll get one of a ton of endings.

7 PUBG: Battlegrounds

The battle royale genre wouldn't exist without a mod of DayZ

Battle royale as a genre catapulted onto the gaming scene thanks to one mod from one modder, PlayerUnknown. He tweaked another ARMA 2 mod into DayZ: Battle Royale, and the seeds of PUBG were sown. The journey to the full-fledged standalone title took a couple of other detours through PlayerUnknown's Battle Royale and H1Z1: King of the Kill before Krafton hired PlayerUnknown to create Battlegrounds, and the 100-to-1 formula was born.

6 Killing Floor

This super popular zombie shooter started out as a mod for Unreal Tournament 2004

Killing Floor started life as a mod for Unreal Tournament 2004, with one objective—mowing down as many zombie-like specimens, or ZEDs, as you can. It's honestly a blast using every weapon you can get your hands on to destroy the creatures, which have waves of reinforcements and special giant bosses to fight off. It's now on its third installment, which will be released in March 2025 for all your horde-destroying fun.

5 Heroes of the Storm

This time, Starcraft 2 got the modding treatment for this awesome MOBA

It took Blizzard several years to bring its own MOBA out, which is honestly surprising since the originals were all created in custom maps from its earlier games. Heroes of the Storm has one hook that you won't find anywhere else: it features characters from across Blizzard's franchises, which is enough to play it all on its own before the deeply engaging gameplay loop. The original was even a mod for Starcraft II, and it took five years from that release for the standalone title to be released.

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4 The Forgotten City

It's Skyrim, but not as you know it

Normally, you'd associate TES V: Skyrim mods with still keeping the role-playing game aspect, but The Forgotten City had grander aspirations. As a mod, it won a national award for the quality of its writing, while also surpassing 3 million downloads. As a reworked, expanded feature game, it is a triumph of story and intrigue, all predicated on one simple point. The literal Golden Rule of "the many shall suffer for the sins of the one," is that if you or one of the other 23 people in the city steal or attack another human, you'll all get turned to gold. But you can go back to the start of the day with every item collected so far if you do get statued, so it's part roguelike and absolutely captivating.

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3 Dear Esther

The Source Engine has a lot to answer for, with this hauntingly beautiful experience

The Source engine has been the basis for many good mods, and Dear Esther was a unique concept when it came out in 2008 as a mod. It's more of a walking simulator, but instead of a simple, but beautiful abandoned island, you're walking through the story as it unfolds in snippets of narration and graffiti. While it started out as a mod, the full release is well worth paying for, and is one of the best examples of video games as an art form that I've experienced.

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