Press [CTRL]+[SHIFT]+[~] ingame to bring up the game console (you should see some green text in the top left of the screen).

To summon one creature without using any souls, type '@ alliwantforxmasisa XXX', where XXX is a particular creature type.

To summon four of the same creatues without using any souls, type '@ aplethoraof XXX', where XXX is a particular creature type.