I'm not up on the statistics of the weaponry, so I'll just post some cool (or "cool") sounding stuff.
Cain's Club
The very weapon used in the first murder.
"Let's go out to the field..."
Good Samaritan
Based on the massive revolver of Hellboy, it would be some kind of sawn-off shotgun-revolver hybrid. As far as cross-overs go at least it fits in with the scenery!
"With this in your right hand, you'll bring doom!" - Hang on, are these all first person? Ha ha.
Shit-Kicking Boots
Steel toe-capped boots. Adds a knock-back attack of kicking to your repertoire of death.
"Comfy!"
Helm of Hades
Allows invisibility at some balanced level, say perfect invisibility while standing still, seventy five percent change of being seen while moving, fifty percent chance of being seen whilst attacking.
"How sophisticated do you have to be to make such a classy, invisible, helmet?"
Matter Cannon
Shove whatever crap you find into it, fire extra heavy cannon balls. Simple.
"Take matters into your own hands!"
Martian Sidewinder
An ancient rifle that fires plasma slugs that bounces off walls.
"Martians musta been interesting guys."
Martian Eradicator
An ancient plasma chaingun that fires through walls, causing radiation poisoning in yourself and foes alike.
"I bet this thing came with a suit...or an exoskeleton."
Martian Talon
An ancient trident which can, but doesn't have to be, powered by plasma to increase it's power.
"Martians, fighting old school before we had schools."
Martian Clicker
The BFG 9000 of the long dead Martians, it would target an enemy with a rocket, but the rocket only explodes when detonated by the player.
"It is harrowing to think that two separate sapient species can be joined in a love of explosive violence."
Oh and to continue adding with beastie:
Quad Shotgun
"Sometimes a man can't wait for a power-up."