St. Andrews Encyclopedia of Theology. This is a fairly new online, free-to-access encyclopaedia of high academic standards, covering the full discipline of theology.
Stanford Encyclopedia of Philosophy. Although focused on philosophy and not theology, many subject cross over, including great thinkers who are considered both philosophers and theologians.
JSTOR. JSTOR provides access to millions of journal articles in dozens of disciplines, including theology. Creating a free account allows you access to a limited number of free articles per month.
Global Journal of Classical Theology. An open-access evangelical theology journal with three annual issues going back to 1998.
HTS Theological Studies. This is an open access journal based out of South Africa that was founded in 1943. It publishes articles from a critical theological perspective on Christian praxis to address local, contextual and international challenges.
Theological Commons. This is is a freely accessible digital library of over 163,000 resources on theology and religion. It was Developed in partnership with the Internet Archive.
Digital Vatican Library. This site provides free access to the Vatican Library’s digitized collections: manuscripts, incunabula, archival materials and inventories.