I once jokingly included in a thesis:
The equations of motion  for a rigid body can be reformulated as…
 I. Newton, Philosophiæ Naturalis Principia Mathematica, 1687.
While citing Newton's work for his equations of motion might be over the top, there is a valid underlying question: what criteria can one use to know when a work is “such a classic” that it doesn't warrant citation anymore.
I'll give example in my field:
- Ewald summation is a neat trick, and very widely used by people who perform molecular simulation of charged species. It was initially published by Paul Ewald in 1921.
- Density functional theory in the Kohn-Sham formalism, published in 1964 (Hohenberg & Kohn) and 1965 (Kohn & Sham).
- Nosé–Hoover thermostat, 1984