You fail to mention in your resources http://repec.org/, one of the oldest open indexes to grey literature (mostly in economics). It is not centralized, but has been around since the last 1990s, and has links between published (paywalled) articles and their corresponding working papers.
You fail to mention in your resources http://repec.org/, one of the oldest open indexes to grey literature (mostly in economics). It is not centralized, but has been around since the last 1990s, and has links between published (paywalled) articles and their corresponding working papers.