Off the top of my head:

Physics:

• Einstein-Hilbert action:

• Expectation values using Feynman's path integral:

• Yang-Mills Lagrangian:

• Deformation quantization (not sure what's the proper name for this, maybe Dirac's rule):

• Fundumental thermodynamic relation:

Mathematics:

• Stokes' theorem:

• Riemann's functional equation:

• L'Hopital's rule:

• Taylor's series:

• Lefschetz-Hopf theorem (unfortunatelly couldn't find one image of both sides of the equation):