A* (A-star)
Pathfinding algorithm that finds the shortest path in a graph — Dijkstra plus a heuristic that steers search toward the goal.
Hart, Nilsson & Raphael (Stanford, 1968). Evaluates nodes by f(n) = g(n) + h(n) where g is the cost from start and h is the estimated cost to goal. If h is admissible (never overestimates) A* guarantees an optimal solution.
Standard algorithm in game AI navigation, robot navigation, GPS routing. Manhattan and Euclidean distance are common heuristics for 2D grids. Competitors: Dijkstra (no heuristic), Jump Point Search (grid optimisation), HPA* (hierarchical).