I just realised that the questions on the site are too mixed up together. On the main page, a question about Newtonian Mechanics is often side-by-side with a question on the standard model/general relativity/string theory etc. I think this is quite messy so a classification would be quite good. Not by tags, but by categories. According to me, the following would be a good classification (but by no means am I suggesting to be final):
- Non-Relativistic Classical Mechanics
- Newtonian Mechanics
- Thermodynamics
- Maybe statistical mechanics?
- Optics
- Wave mechanics and oscillators
- Hamiltonian Mechanics
- Lagrangian Mechanics
- Newtonian Gravitation
- Maxwellian Electromagnetism
- Relativity
- Special Relativity
- General Relativity
- Alternatives to General Relativity (Einstein-Cartan theory, etc.)
- Quantum Mechanics
- Old Quantum Theory
- Heisenberg Matrix mechanics
- Schrodinger Wave mechanics
- Feynman Path Integral mechanics
- Scattering
Quantum Field Theory
- Relativistic Quantum Mechanics (Klein-Gordon Equation, Weyl Equation, Dirac Equation, etc.)
- Quantum Chromodynamics
- Electroweak theory
- Quantum Electrodynamics
- Higgs theory
- Standard model
Quantum Gravity and Unification theories
- String theory
- Loop Quantum Gravity
- Maybe Kaluza-Klein?
- etc. (GUTs, preon models, and other attempts to Quantum Gravity)