  By means of quantum Monte Carlo simulations we study phase diagrams of
dipolar bosons in a square optical lattice. The dipoles in the system are
parallel to each other and their orientation can be fixed in any direction of
the three-dimensional space. Starting from experimentally tunable parameters
like scattering length and dipolar interaction strength, we derive the
parameters entering the effective Hamiltonian. Depending on the direction of
the dipoles, various types of supersolids (e.g. checkerboard, stripe) and
solids (checkerboard, stripe, diagonal stripe, and an incompressible phase) can
be stabilized. Remarkably, we find a cluster supersolid characterized by the
formation of horizontal clusters of particles. These clusters order along a
direction at an angle with the horizontal. Moreover, we find what we call a
grain-boundary superfluid. In this phase, regions with solid order are
separated by extended defects -- grain boundaries -- which support
superfluidity. We also investigate the robustness of the stripe supersolid
against thermal fluctuations. Finally, we comment on the experimental
realization of the phases found.
