A work in progress. Over 700 pages, encompassing band structure, symmetry and much more.
By Daniel Arovas, UC Santa Barbara.

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).