Of the games I regularly play, D&D has enough resources, both on- and off-line, that I can find pretty much whatever I’m looking for with a fairly quick Google search. It makes sense, though. D&D is one of the oldest RPGs and probably has the largest player base of any RPG in the world. Ever since I started using the Internet in the 1990s, I’ve been able to find online resources for D&D (and there certainly was no lack of printed resources back then).
I gotta give a shout-out to the fan efforts to support WEG’s D6 Star Wars RPG and the fan supporting Goodman Games’s Dungeon Crawl Classics. There’s a ton of high-quality resources online for both of those games