  We describe a new algorithm for the numerical simulation of quantum spin and
boson systems. The method is based on the Trotter decomposition in imaginary
time and a decoupling by auxiliary Ising spins. It can be applied, in
principle, to arbitrary (random) spin systems, however in general it suffers
from the ``minus-sign problem''. This problem is absent in the case of the
Ising model in a transverse field in arbitrary dimensions and geometries. We
show test results for the spin--1/2 XY model, the one--dimensional transverse
Ising model with disorder, and the phase transition induced by a transverse
field in the two-dimensional Ising model.
