In the spirit of Talk to the Skull and Shrunken Head Scrolls, let's bring on the value of monster heads.
Monsters can be worth salvaging parts. In GURPS Dungeon Fantasy, the usual rules apply - specific monsters may have ichor, tusks, hides/furs, poisons, etc. that can be recovered from the corpse. But for some creatures, the head is especially useful.
How to use it?
As a Weapon - Some monsters with lethal gazes can retain this gaze after death. How long is variable - it can range from seconds to minutes to hours or just work forever. When in doubt 1d hours is reasonable.
As a Power Item - Spell-casting or magically powered monster heads can retain their mana or magic source. Typically this mana is aspected, and can only be used for certain spell colleges. The head of an ifrit might be only useful for fire spells, the head of a slorn for animal spells (possibly only vs. reptiles), the head of a human for mind-control spells (perhaps the head of a wizard only for meta-magic spells). These are a non-rechargeable and can be used concurrently with another power item (casters are still restricted to one external power source at a time). A good guideline is that they provide power equal to half their original FP.
In D&D-style games, they may retain a few spell-levels of power (or function as scrolls). In Rolemaster, they act as a PP pool separate from the caster's own. In either case, unless shrunken and preserved, they will decay normally and lose their vitality.
As a Knowledge Source, either for magical questioning (Summon Spirit works on the head in my games), mechanical examination (post mortem phrenology), or post-death psionic brainscan. Or the head may remain animate, like Gacioch the demon in Glen Cook's The Swordbearer, just because a demon's body is destroyed it doesn't mean the head can't stick around and cause more excitement. A demon doesn't need a body to connive, confuse, or corrupt.
As ingredients - Ground up head bits might not do it. Maybe you need the whole ice weasel head, or unicorn head, or carnivorous ape head for your potion recipe.
As bounty. Sometimes, the sign that says "orc heads, 1 gold piece bounty each" doesn't mean "orc ears." They want the whole damn head, samurai style. This is especially true if someone wants it for their wall!
Finally, some heads are just dangerous. A troll's head is just going to keep after you until you burn it, a wight's head might still enervate its foes, a poisonous mushroom man's head is still poisonous. Not everything is loot . . .