Lectures on theoretical physics from Cambridge University professor David Tong.

David Tong at Cambridge is a gifted educator and communicator who has written lecture notes that span a wide swath of the physics curriculum, from introductory material on mechanics through advanced graduate-level treatments of quantum field theory. Truly, these are a fantastic resource, made freely available.

Link and quote thanks to Douglas Natelson (nanoscale views).

I particularly like his notes on solid state physics (which contains links to other resources).