  Strong electron correlations and interference effects are discussed in
parallel-coupled single-level and orbitally doubly degenerate quantum dots. The
finite-U mean-field slave boson approach is used to study many-body effects.
The analysis is carried out in a wide range of parameter space including both
atomic-like and molecular-like Kondo regimes and taking into account various
perturbations, like interdot tunneling, interdot interaction, mixing of the
electrode channels and exchange interaction. We also discuss the influence of
singularities of electronic structure and the impact of polarization of
electrodes. Special attention is paid to potential spintronic applications of
these systems showing how current polarization can be controlled by adjusting
interference conditions and correlations by gate voltage. Simple proposals of
double dot spin valve and bipolar electrically tunable spin filter are
presented.
