The Triumph of the Archangel Michael over Satan is the meme.
http://en.wikipedia.org/wiki/Saint_Michael_(Roman_Catholic)

There are quite a few versions of Michael defeating Satan; however, to my knowledge, the Navarre Picture Bible is the only version that clearly depicts the weapon as having a spear head.

Dating from 1550, the Geschlechterbuch der Stadt Augsburg can hardly be said to be from the Middle Ages, no matter what definition one uses for "medieval". Still, it depicts some absolutely fabulous arms and armour, and many examples of exotic, fantasy-type arms that fit perfectly with the theme of this thread. Really, the Geschlecterbuch deserves a thread all of its own, but here's just a few examples of the many extraordinary illustrations that it contains:

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Source: http://daten.digitale-sammlungen.de/~db/bsb00...pdfseitex=

The knight in the upper left, who's tipping up the visor on his helmet, has a rather odd looking weapon. Image from the Queen Mary Psalter.

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Source: http://manuscriptminiatures.com/4154/12090/

I've posted this elsewhere, but here it is again, from the Roman de Brut, Edward III: Destruction de Rome.

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Source: http://manuscriptminiatures.com/4100/12707/

Another interesting glaive, from The Taymouth Hours.

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Source: http://manuscriptminiatures.com/4181/10662/

This is such an interesting thread. Great topic idea and fantastic pictures.

I think this example is probably just an artistic error, rather than attempting to represent an exotic weapon. Nevertheless, the monstrous figure in this manuscript illustration appears to be holding a sword/club hybrid. It's from Artus de Bretagne.

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Source: http://manuscriptminiatures.com/5207/16900/

The weapon on the left here looks like one in the Queen Mary Psalter. Here, it looks like a wooden club with a sharpened, metal edge and hook.

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Source: http://manuscriptminiatures.com/4071/7164/

The polearm in the background here looks like a military fork with a back spike and a fluke. This is Speculum humanae salvationis, imperfect.

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Source: http://manuscriptminiatures.com/4177/13102/

Another image that has been posted elsewhere, this two-handed sword seems like a strange form of falchion.

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Source: http://manuscriptminiatures.com/4373/7073/

In this case, the weapon carried by the front knight has a name: it seems to be a plançon à picot. Given how rarely they seem to be represented in art- this is the first one I've seen- I've included it just the same. You can read more here: http://en.wikipedia.org/wiki/Plan%C3%A7on_a_picot

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Source: http://manuscriptminiatures.com/4013/11407/

That fantasy falchion looks like another fantasy sword... orcrist!

It can be easy to forget that fantasy weapons need not be exotic nor unusual. By taking an ordinary weapon and increasing its size such that it is a hyperbole of real life weapons, one can create a fantasy weapon. The illustrator of this manuscript seems to have fairly closely followed the Biblical description of Goliath's massive spear, and depicted one with similarly massive proportions. This comes from BL Royal 6 E VI Omne Bonum, A-D.

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Source:http://manuscriptminiatures.com/4171/13090/

The far right soldier in the front carries a very odd weapon. When I initially looked at it, I thought the weapon was supposed to be a javelin with fletches, shown with the back end facing forward, and the artist mistakenly coloured the fletches and haft grey. But, looking closer, this interpretation does not make sense; the grey coloured section looks completely wrong as a javelin with fletches. The weapon looks vaguely reminiscent of a badly drawn winged spear, or a similarly distorted partizan, but neither of these fit either. Perhaps it is some sort of bludgeoning weapon? Alternative interpretations are more than welcome. This comes from the Morgan Weltchronik.

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Source: http://manuscriptminiatures.com/4693/12807/

Considering half the point is buried in the downed man's ass cheek, I suspect it's the exact same as the spear on the far left.

I believe that it's just a normal spear but the head is buried partly in the corpse. It's consistent with the way spears are depicted in the manuscript.

http://manuscriptminiatures.com/media/manuscr...850-24.jpg

Perhaps it should not be considered a fantasy piece, but the hafted weapon to the far left is fairly unique. I wondered if it might be an instrument of torture, given the context of this image. However, the same weapon shows up again in the manuscript, and the second time, it is unambiguous that it is a weapon, not an instrument of torture. In form, its similar to a war hammer, save that it does not have a flat face for striking. This is from Politica & Economica.

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Here's the second image, showing it clearly in a military context:

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Sources: http://manuscriptminiatures.com/4953/14963/ and http://manuscriptminiatures.com/4953/14965/

Hmm... I had not realized that the weapon was supposed to be stuck in a person, but now that you mention it, that interpretation makes sense. Oh well.