  Skew scattering of electrons induced by a spin cluster is studied
theoretically focusing on metals with localized magnetic moments. The
scattering probability is calculated by a non-perturbative $T$ matrix method;
this method is valid for arbitrary strength of electron-spin coupling. We show
the scattering of electrons by a three-spin cluster produces a skew angle of
order $0.1\pi$ rad when the electron-spin coupling is comparable to the
bandwidth. This is one or two orders of magnitude larger than the usual skew
angle by an impurity with spin-orbit interaction. Systematic analysis of the
scattering probability of one-, two-, and three-spin clusters show that three
spins are necessary for skew scattering. We also discuss the relation between
anomalous/spin Hall effects and the spin chiralities; we find that the spin
Hall effect requires three spins while it is related to the vector spin
chirality defined by a pair of spins. The relevance of these results to the
large extrinsic anomalous and spin Hall effects in noncentrosymmetric and/or
frustrated magnets is also discussed.
