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.