Open access e-print archive in philosophy. Formerly know as the PhilPapers Archive.
Highly respected open source encyclopedia of philosophy.
The main commercial reference source on philosophy.
Scholarship (articles, books, dissertations and book reviews) in philosophy.
Full-text literature, philosophy, and theology classics online in scholarly editions, in addition to the correspondence of selected authors.
Landmark scholarship (articles, books, dissertations and book reviews) in philosophy. Curated by a community of philosophers. Very good for exploring the study of philosophy by topic.