There are A LOT of free RPGs out there, from the cool and intricate to the tiny and niche (or often both). However, the free one that sticks in my mind the most, that I’ve actually played, is Wushu.
Wushu is action roleplaying where really only one rule matters - the rule of cool. The cooler you make your descriptions of what you do, the more successful you are likely to be. It’s extremely light, mechanics-wise, and is tightly focused on making cool scenarios with ridiculous, over-the-top action scenes the likes of which wouldn’t look out of place in crazy kung-fu films and action movies. Because character generation is a simple list of ‘traits’ you make up and there are no real stats, per se, it super easy to get started and run.
I ran a game of Wushu a long time ago using the Feng Shui setting - players started off in the past and had to save the future, or something along those lines. Lots of crazy fights with laser-shooting mecha-dragons and fighting mooks on logs floating down rapids. That kind of thing.
If you’ve never tried Wushu, I really recommend you check it out if you want some light, fun, high-action gaming.