  We propose a new ansatz for the ground-state wave function of quantum
many-body systems on a lattice. The key idea is to cover the lattice with
plaquettes and obtain a state whose configurational weights can be optimized by
means of a Variational Monte Carlo algorithm. Such a scheme applies to any
dimension, without any "sign" instability. We show results for various two
dimensional spin models (including frustrated ones). A detailed comparison with
available exact results, as well as with variational methods based on different
ansatzs is offered. In particular, our numerical estimates are in quite good
agreement with exact ones for unfrustrated systems, and compare favorably to
other methods for frustrated ones.
