Wiki: Mercenaries

Varangian Guard site (Posted by some other member on these boards in another thread. Can't recall whom and which thread)

Swiss Guards

Condottieri. The leaders of mercenaries in Italy

Almogavares

Catalan Company

Otherwise, from what I've read many medieval kingdoms hired mercenaries, but the most prominent are probably Byzantium, Italian city-states & the taifa kingdoms of Al-Andalus.