From JSTOR:
The launch of a new “Open Content” page that easier exploration of all the open content on the JSTOR platform.
JSTOR currently includes a growing collection of 2,000 open access ebooks, more than 500,000 early journal articles in the public domain, and a range of other journal content made freely available by our partner publishers.
This combined group of open and free content is fully searchable and accessible to anyone in the world. All free and open content on JSTOR includes downloadable full-text PDF files and is available along with free research tools on the platform.
Read the Complete Blog Post
UPDATED October 26, 2017 Coming Soon: Brill Will Offer Open Access Ebooks on JSTOR