Mendeley now has an open access search filter check-box for its catalogue search. Although unfortunately it currently considers JSTOR articles to be open access. For most of us they’re not, beyond the first page.

medeley-oa

Europeana has the same problem, containing linked records pages for JSTOR content which isn’t open and public (e.g: “History of the churches of India” – Europeana links to it, it’s public domain, but JSTOR has the only copy and wants $10 to access it).