The nail clipper in Bad Day LA (I'm pretty sure I'm the only person who likes that game...)
And the severed cow head in Postal 2. Apocalypse weekend had some very amazing stupid weapons too, like the boomerang machate and a scythe. Man that game is addictive, I want to play it again.
And then there's that weird weapon in Cave Story+ which degrades as you level it up, in the end shooting rubber ducks which do 1 damage.
I've also seen some indian videogame having a cow as a sidekick... At least in the video it was (it crashed through a wooden wall sorta thing, Kool-Aid style), but now Im on my way to look that game up 'cause I'd be stupid not wanting to play that shit.