Below are some links to physics related content on the internet which I have found useful.

1. The Inspire database, containing information about HEP researchers and their publications.

2. The arXiv, giving free access to physics preprints.

3. Shiraz Minwalla's video lectures on string theory given at TIFR.

4. Lecture notes on the AdS/CFT correspondence by Aharony, Gubser, Maldacena, Ooguri and Oz.

5. Paul Ginsparg's lecture notes on conformal field theory.

6. David Tong's lecture notes on quantum field theory and string theory.

7. Matthias Blau's lecture notes on general relativity.

8. Daniel Baumann's TASI lecture notes on cosmology.

9. Edward Witten's lecture notes on causal properties of general relativity.

10. Pavel Kovtun's lecture notes on hydrodynamics.

11. Lecture notes on the Schwinger-Keldysh approach to hydrodynamics by Hong Liu and Paolo Glorioso.

12. Gerald Dunne's lecture notes on Chern-Simons theory.