  Two different kinds of metal transition oxides have been studied for their
large thermopower values. The first one corresponds to the Tl-based misfit
cobaltite which is a hole-doped metal. We demonstrate that the partial
Bi-substitution for Tl in this phase induces an increase of the room
temperature (RT) thermopower (TEP) value. Same result is obtained with the new
Pb_{1/3}SrCoO_{3+delta} misfit corresponding to the Tl complete replacement by
lead. Simultaneously, the T dependence of their resistivity exhibits a
re-entrance below 70-90K where a large negative magnetoresistance is observed.
Magnetic measurements reveal a strong interplay between spins and charges for
this class of materials. Electron-doped (n-type) perovskite manganites are a
second class of potential candidates for applications. In particular, the
Ru^{4+/5+} substitution for Mn in the CaMnO_3 semi-conductor induces a drastic
drop of the resistivity values. Metals with large RT TEP values and not too
large thermal conductivities are generated. A comparison with best known
materials, Bi_2Te_3 and NaCo_2O_4 is made.
