The Feynman Lectures on Physics
feynmanlectures.caltech.edu
The complete text of Richard Feynman's iconic undergraduate physics lectures, freely available online in a beautifully typeset HTML format.
Why it is useful
These lectures are widely considered the best introduction to physics ever written — Feynman's ability to build physical intuition rather than just procedural calculation skills is unmatched. The free online version is fully searchable with properly rendered equations. For anyone studying physics or wanting to understand the universe with genuine depth, this is the primary text.