The halberd was one of the first polearms, and it was by far the most common one. This weapon is sometimes called the halbert or poleaxe.
The basic halberd is an axe, having a six foot long ash handle, with a spearhead mounted at the end of the pole. The axe head itself is often balanced by a hook on the opposite side. Giving this weapon three distinct types of attacks.
^ --Spearhead
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\ /# \ ---Hook
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Axe Head #
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Pole shown much shorter to conserve space.
The main function of the halberd was that of an anti-cavalry weapon. The spearhead was used to stand against the initial charge, while the hook is used to dismount the horseman. Finally, a mighty swing of the axe head could finish off the freshly unmounted enemy, cutting through most armor with ease. The halberd is best used in a group, as a soldier taking a full swing with one is completely unguarded, and must rely on his allies for protection. A halberdier unit would usually inflict huge casualties on enemy horsemen, while receiving huge casualties in return. Because the relatively short length of their weapon allowed for opposing knights to lance many halberdiers before they could inflict any damage. The awl pike was eventually adopted to help even the score a little more.
There are several online retailers that sell medieval halberds. In a quick search I was able to find both authentic servicable halberds ($184 USD), and decorative fantasy halberds ($89 USD). I am not responsible for anyone you may skewer with one of these.