Once I beat the game, I unlocked one cheat - decapitation. I didn't want to play the game nine more times to get the other cheats so I looked up an unlock code on the internet. This unlocked all ten cheats.

By the way, the code at GameFAQs is wrong... you have to input it at the title screen. It doesn't mention it at all.

In a nutshell:
Decapitation - headshots decapitate enemies
No reload / Infinite grenades / ammo - self-explanatory
Molotovs - Grenades turn into Molotovs.
Walking Enemies - Enemies walk, don't run. Doesn't work in sniper levels - if everyone walked in 'em then the stages would take AGES to finish.
Drug effect - crappy motion blur effect.
Super shotgun - a slightly more powerful shotgun blast which knocks the enemy back.
Exploding enemies - kill an enemy and it goes boom!

All music used under fair use, please don't sue me, kthx.

(And the title is a reference to Highlander 2... in case you're confused or something.)

(Reuploaded. It appears that the source AVI I used had -two- audio tracks for some stupid reason!) (less info)