"The Association of Religion Data Archives (ARDA) strives to democratize access to the best data on religion. Founded as the American Religion Data Archive in 1997 and going online in 1998, the initial archive was targeted at researchers interested in American religion. The targeted audience and the data collection have both greatly expanded since 1998, now including American and international collections submitted by the foremost religion scholars and research centers in the world." (from their website)

Catholic Research Resources Alliance

"The CRRA provides enduring global access to Catholic research resources in the Americas, beginning with access to the rare, unique and uncommon research materials."

CRRA has many wonderful digital resources:

• The portal to Catholic research resources in the Americas, with attention to unique and uncommonly held materials;
• A freely available digital newspaper collection in the Catholic News Archive;
• An extensive range of subject guides exploring topics of Catholic interest

Google Scholar

Results may be rather STEM-heavy (Science, Technology, Engineering, Medicine). "Google Scholar index includes peer-reviewed online academic journals and books, conference papers, theses and dissertations, preprints, abstracts, technical reports, and other scholarly literature, including court opinions and patents." (from Wikipedia)

The Internet Archive

Most famous for The Wayback Machine, which preserves older versions of websites, the Internet Archive "... is building a digital library of Internet sites and other cultural artifacts in digital form. Like a paper library, we provide free access to researchers, historians, scholars, people with print disabilities, and the general public. Our mission is to provide Universal Access to All Knowledge." (from their website)

The Open Access Digital Theological Library

"The mission of Open Access Digital Theological Library (OADTL) is to curate high-quality content in religious studies and related disciplines from publisher websites, institutional repositories, scholarly societies, archives, and stable public domain collections. ... The OADTL is staffed by professional librarians and curates content without regard for theological or confessional perspective. It is hoped that the increased access to high-quality religious studies content will serve scholars and students of religion."

Project Gutenberg is a library of over 70,000 free books.

"Everything from Project Gutenberg is gratis, libre, and completely without cost to readers. ... Project Gutenberg eBooks require no special apps to read, just the regular Web browsers or eBook readers that are included with computers and mobile devices." (from their website) Some of these results will show up in the Everything search in the library catalog.