A Spot of Friendly Wizardry is a set of free game rules for duels between two wizards.
Tiny Wars Fantasy Skirmish is a set of free wargames rules for 15mm figures on a small tabletop.
Featherstone’s Ghost Legions is a set of free wargames rules for using historical figures to fight battles between the dead in the underworld.
The Jackson Gamers present their Medieval version of the venerable Rules by Ral.
Here’s a free, printable pdf of the core spellbook for Frostgrave