There are a lot of free RPGs out there, many of them awesome, so I’ll be really curious to see what people write about today. But for me, one of the granddaddies of free RPGs has yet to be surpassed, and remains one of the few existing games I still use to build new games (rather than building from scratch): Fudge. By being almost more of a framework than a game system, it gives me the tools to do whatever I want, and is the perfect scaffolding to build the game I want if something in existence doesn’t already do it for me.
Eliminate stats, create some specific skill lists, add hero points, and make word-based, on-the-fly modifiers a core part of gameplay, and you end up with Fate.
You can create something every bit as crunchy as Hero System, with multipliers and scaling and multiple forms of damage and armor.
Drop a die and invent Keys, and you have The Shadow of Yesterday.
Streamline that a little bit further, throw in a new way to handle skills, and switch over to a dice pool system, and you end up with Lady Blackbird. (Though while the pedigree of Lady Blackbird is obvious, I’d actually argue that changing to a non-bellcurve, non-zero centered die rolling mechanism is the one change you can make that makes it “no longer ‘Fudge’.”)
Fudge was one of the systems we used for Ogalepihcra, and definitely the easiest one to adapt.
You could probably even turn Fudge into a true Story Now system by using stats that focused on goals and conflicts and relationships, rather than skills and capabilities.
If you like Fate, you should check out Fudge, particularly Five-Point Fudge. If you like Fate, give it’s ancestor a look. If you don’t like Fate, you maybe should still give Fudge a look, depending on what you don’t like about it. Most instantiations of Fudge are significantly less complex/crunchy than Fate, so if all the moving pieces are the problem, Fudge might be your answer. Many instantiations of Fudge are also more “traditional”, in that they’re primarily about attributes and skills, rather than aspects or relationships or any of that fancy abstract and/or Narrativist stuff. Me? I love abstract and story-focused RPG rules. But if you don’t like Fate because of those elements, Fudge might be for you. If you like the consistency of Hero System or GURPS, but not the complexity, Fudge is a perfect toolkit for you. Finally, if you want to build your own game, but don’t want to do it completely from scratch, Fudge gives you a solid foundation to build upon.