Self-propelled particles: each moves at constant speed and aligns its direction with neighbors within radius r, plus angular noise η. At low noise, collective motion (flocking) emerges — a non-equilibrium phase transition with no analog in equilibrium statistical mechanics.
Vicsek et al. 1995: this simple rule produces a continuous (or weakly discontinuous) order-disorder transition as noise increases. Order parameter φ = |⟨e^{iθ}⟩| = 1 for perfect alignment, 0 for disorder. Analogous to ferromagnet but out of equilibrium.