Younger gamers might not realize this, but when WotC/Hasbro released Dungeons & Dragons 3rd Edition in 2000, it was a huge change in gaming. Here was a game where the vast majority of the rules (the d20 System) were available for free online (legally!), and they had a license system set up where anyone could publish compatible content for their game. Dozens of small press companies popped up to take advantage of this, publishing all manner of campaign settings and class & race guides and bestiaries and spell & equipment catalogues and et cetera, of varying degrees of quality. It really hleped breathe new life into the gaming industry (even if many of those small press companies folded after a few years), and I enjoyed seeing the wealth of stuff we were getting. And, some companies did make it long enough to go on to tweak things and make their own games/rules, like Green Ronin did with Mutants & Masterminds.
On a related note, speaking of superhero games, back in my late teens/early 20s I’d spend hours translating characters from TSR’s Marvel Super Heroes to Mayfair’s DC Heroes, and vice verse. Never really did that very much with any other superhero RPG system, though.