Of all the super-hero movies around, "Darkman" is my favourite. An original creation from Raimi, and not based on any comics, "Darkman" follows the story of Dr. Peyton Westlake, a scientist about to make a breakthrough in synthetic skin. Peyton's girlfriend, Julie Hastings is a hot-shot lawyer who gets on the wrong side of Robert G. Durant. Durant and his henchmen destroy Peyton's lab, and leave him for dead. Horribly burned and scarred beyond recognition, Peyton uses his uncompleted and synthetic skin to get revenge on those who wronged him - he becomes Darkman.
Darkman has no real super-powers - apart from some increased strength and his inability to feel pain - he uses the synthetic skin to assume the alternate identities, and fights his battles that way. Darkman is not a real super-hero. He makes no pledge to rid the world of evil-doers. He is just out for revenge, and every time Darkman acts, Darkman's dark-side claims just a little bit more of Peyton.
Others have mentioned the parallels to 'Robocop'. I'd like to mention the parallels to Sam Raimi's recent efforts with 'Spider-Man' - the two are very similar style-wise, but Peter Parker/Spider-Man is generally pretty happy and cheerful, where as Darkman finds sadistic pleasure in the way he toys with his enemies before finishing them off. 'Spider-Man' is family-friendly, 'Darkman' is not the sort of movie you would want to show to parents or younger-siblings.
'Darkman' is original, and yet another under-rated classic from Sam Raimi. Check it out if you like Sam Raimi, but avoid if you enjoy the family-friendly nature of 'Spider-Man'. Definitely my favourite super-hero movie, and nearly up there with Tim Burton's 'Batman' and 'Batman Returns', the best super-hero movies ever.