The Post-Reformation Digital Library looks to be an enormously valuable resource. Nick Batzig says “This is the most comprehensive collection of free online PDF theological resources. It will be, without a doubt, a massively important site for those interested in pre-20th Century studies.”
There is an enormous amount of helpful theological primary sources available on Google Books, Internet Archive, and other sites. The PRDL helpfully organizes these by category: Reformed, Lutheran, Roman Catholic, Anabaptist, Arminian-Remonstrant, and Socinian-Unitarian. Also included are smaller sections on Early Modern Philosophy and Patristic and Medieval Philosophy.