Sulfidation is relevant to the formation of sulfide minerals.
A large scale application of sulfidation is the conversion of molybdenum oxides to the corresponding sulfides.  This conversion is a step in the preparation of catalysts for hydrodesulfurization wherein alumina impregnated with molybdate salts are converted to molybdenum disulfide by the action of hydrogen sulfide.
In organosulfur chemistry, sulfiding is often called thiation.  The preparation of thioamides from amides involves thiation.  A typical reagent is phosphorus pentasulfide (PS).  The idealized equation for this conversion is:
:RC(O)NH  +  1/4 PS   →   RC(S)NH  +  1/4 PSO   
This conversion where an oxygen atom in the amide function is replaced by a sulfur atom involves no redox reaction.