There’s a lot of stuff on Researchgate. And with the evolution of European grants, there are a few publicly-available repositories, like hal.science (funded by the French government and the default repository for public research in France, I think you have to be with some kind of research institution so it’s not quite as open as arxiv but there are plenty of good articles there).

It is also valuable for scientists as it is often a 'directors cut' version of the paper. Journal submissions are heavy edited and shortened to fit into the page limits.

I am thankful for arXiv only made minor adjustments to its UI over the years, and I hope arXiv keep it that way.