Stanford Encyclopedia of Philosophy
plato.stanford.edu
A comprehensive, peer-reviewed online encyclopedia of philosophy maintained by Stanford University — free, authoritative, and continuously updated.
Why it is useful
Each entry is written by a leading expert and peer-reviewed, making it as authoritative as any academic publication while being freely accessible. Unlike Wikipedia, the entries engage seriously with the technical literature and are updated as fields develop. For anyone who wants to actually understand what philosophers argue about and why — rather than a 100-word summary — it is the only reliable free source.