H-Hoac is the H-Net network for Historians of American Communism.
This all looks fantastic in terms of cost (seemingly free), scope (large number of documents scanned), and organization (very easy to locate single documents down to the list level. The problem, however, is that the viewable documents seem to be blown-up thumbnails — completely illegible. Not sure if I am doing something wrong, they are doing something wrong, or there is a paywall I don't know about that allows legibility.
Regardless, it looks like a big step forward is being made, now or soon.