"As I shifted into broader machine learning, arXiv became my new home." Similarly, when I researched machine learning topics for my master thesis (around 2001)

"As I shifted into broader machine learning, arXiv became my new home." Similarly, when I researched machine learning topics for my master thesis (around 2001), citeseer.org proved to be an absolute treasure trove. I did not check out anything beyond machine learning and general computer science topics, but for these at least open access papers have quite a bit of tradition by now.