Flow field
A vector field over the grid where every cell points to the next step toward the goal, so hundreds of units can share one route without searching individually.
Instead of running A* once per unit, the field is computed once for a destination: first a cost map, then an integration map via a Dijkstra-like sweep outward from the goal, and finally a direction vector in each cell toward its cheapest neighbour. Every unit then does a single lookup and follows the arrow. The cost becomes essentially independent of how many units there are.
The approach suits crowds and large-scale strategy and broke through with the pathfinding in Supreme Commander 2 (2010); its academic ancestor is "Continuum Crowds" by Treuille, Cooper and Popović (2006). The drawback mirrors the strength — a field is tied to one goal, so many distinct destinations need many fields or a different method.