An Hour of Wolves and Shattered Shields has a selection of high resolution shield designs for Vikings. Either print them out on decal paper, or on regular paper and glue them to the model.
dark ages
Ravenfeast – Norse Saga Skirmish Rules
Ravenfeast is a set of free wargames rules for skirmishes in the Norse sagas.
Shieldwall Dark Ages Rules
Legio Wargames offers Shieldwall, a set of free wargames rules for Dark Ages battles in the British Isles and Northern Europe.
White Steeds and Seax
White Steeds and Seax is a variant of the Dark Age Infantry Slog rules, which include rules for cavalry.
Viking Painting Guide
Crusader Miniatures has a painting guide for Vikings.