The sample can be anything such as a guitar riff from another song or something taken from a movie.

hmmm......Acid Bath: Cassie Eat's Cockroaches, sample: from clockwork orange........Wumpscut, samples: brad pitt and sigorney weaver

Cradle of Filth: Krieg Kindersoldaten (Adolf Hitler and Josef Goebbels samples)
Marduk: Slay the Nazarene (Martin Luther King sample)
Mortician: Hacked up for barbecue ( the drowning girl and the chainsaw...)
some Impetigo samples....

Anaal Nathrakh : The Codex Necro : The exorcist sample sort of comes out of nowhere for no reason, almost making it quite humorous. Heavy Metal Heavy Metal Heavy Metal Heavy Metal Heavy Metal Heavy Metal Heavy Metal Heavy Metal Your mom sux cock in hell Heavy Metal Heavy Metal Heavy Metal Heavy Metal Heavy Metal Heavy Metal Heavy Metal Heavy Metal

I always thought that sample in "And When He Falleth" by Theatre of Tragedy was very cool.

The Exorcist has been used by so many metal bands it's almost redundant to list any, so I won't.

Skinny Puppy sampled the movie Plague Dogs in the beginning of "Testure". "I hope you make sure we're properly dead before you start, old Rip-beak." Sets the mood nicely. Brutal Truth used a truncated version for "Walking Corpse" that also set up the song.

Not to blow my own horn too much, but in the Seventh Image song "Carpal Tunnel" we used a sample from a very old Val Kilmer film. Just after the bombastic and somewhat chaotic intro an old man says, "This is NOT Mel Torme!". Then the first verse hits.

I like the "Bomb the living beejus outa of those forces" in KMFDMs "Drug against war"

The "Go have sex with jesus christ you f-f-faget" bit from Agoraphobic Nosebleeds "Built to grind" always makes me laugh because it just..has no place whatsoever on that album and is pretty much half the song's length.

The Meat shits had a cool sample in the "There is no God", one of the few samples that just flowed perfectly into the song.

There's more I just can't remember them off-hand