Kings of War is a new set of fantasy rules from Mantic Games, who make some very cool figures. But the rules are non-figure-line specific. And they’re free.
From Impact Games, Elfball is a set of fantasy “football” rules. It’s played on an oval field, so it’s not like “American Rules.”
Plastic Attack is a simple set of free wargames rules for playing games with your plastic action figures.
The Fantasy Trip Third Dimension is a set of rules modifications for The Fantasy Trip / Melee / Wizard that allows you to use Heroscape sets instead of cardboard counters. The three dimensions create some issues, but these solve them. You’ll need The Fantasy Trip to play, but if you don’t have a copy, you can make do with Legends, which is free from Dark City Games.
Melee and Wizard is a set of rules modifications to allow you to use The Fantasy Trip / Melee / Wizard for mass combat.