WorldCat - Find books, e-books, book chapters, articles (including free full texts in some cases), theses/dissertations, and more.
Google Custom Search of Theological Journals: “Targets over 1,000 Web sites/pages to retrieve full text articles from hundreds of open access journals and other periodicals related to religious studies, scripture studies, systematic theology, practical theology, and cognate disciplines.”
JURN: an academic search engine with a unique strength in the arts and humanities allows you to search for free scholarly articles, chapters, theses, images. Some of the websites (e.g., academia.edu) may require you to create a free account in order to download the full text.
Directory of Open Access Journals (DOAJ) is "an online directory that indexes and provides access to high quality, open access, peer-reviewed journals." Because this site includes journals in all disciplines, your will have to search for the few that have to do with religion. Tip: select "Philosophy, Psychology, Religion" under Browse Subjects to obtain more relevant results and try including in your search the names of individual books of the Bible and "commentary" or "exegesis."
GlobeTheoLib - Online digital library of journals, books, and other materials relating to theology, interreligious dialogue, ethics, and ecumenism in World Christianity. Indexes over 120 journals, most of which are published free online. Note: some full-text articles require you to register for a free account.
Free e-Books in Biblical Studies (Google Doc): Lists a great number of items available online, mostly in PDF format from Google Books. Organized by subject.
Christian Classics Ethereal Library: "The CCEL is a digital library of hundreds of classic Christian books selected for edification and education.” This link connects to their advanced search page.
The Theological Commons is a digital library of 79,332 open access books and periodicals on theology and religion, including 29,730 volumes from the Princeton Theological Seminary Library. The collection spans the period from -1800 to 1926+ and covers subjects such as Protestantism, History, Catholic Church, Church History, and more. Formats include books, periodicals, theses, and manuscripts in multiple languages. The material can be read online, or downloaded in PDF, or sent to Kindle.
HathiTrust Digital Library. HathiTrust is a partnership of academic & research institutions, offering a collection of millions of titles digitized from libraries around the world.
Bible Commentaries at religion-online.org: Provides direct links to the full texts of books and chapters by recognized authorities. Works are cateogorized by subject, indexed by author and title, and full-text searchable.
Project Gutenberg: The first and largest single collection of free electronic books, or eBooks.