Ok, so I've heard Warhammer who were great and Apocalyptic Raids, who weren't quite as cool but were still pretty enjoyable. I know this is probably a pretty small scene if you will, but I'm keen on getting some more hellhammer worship.

If Celtic Frost worship would suffice, check out Nordic Mist.

Coffins! Although they are not a straight clone band if that's what you wanted.

Gallhammer
Winter
and to a lesser extent, Runemagick

I was looking for straight clone bands, but will be happy enough taking bands that sound somewhat similar.

Uhm, the swedish Tyrant. There's a bit of Venom in all that Hellhammer worship, but there was a bit of venom in hellhammer, so whatever.

Slaughter (the Glam band, obviously). A bit of Voivod in the sound, but otherwise it's just punishing, bass-heavy Hellhammer worship with the intensity set to 11.

If you haven't yet heard the album, Samael's Worship Him is, by far, their most Hellhammer-obsessed album. The song Into The Pentagram is an obvious tribute to Hellhammer's Triumph of Death. The vox, however, are not anything like the grunts of Tom G. They're more of a low howl (not a shriek, though Vorphalack will occasionally break into a shriek at the end of some words). Oh, and the music is a bit more doomy and mid-paced than Hellhammer.

Vomitor (of Austrialia) is similar to Slaughter in that they worship both Hellhammer and early Voivod. They don't sound a thing like Slaughter, though. Much faster and more thrashy.

Usurper and Scepter (both of Chicago) are both basically straight up hellhammer/Celtic Frost clones (like Apocalyptic Raids, but more so).

I've always thought of tha-Norr (of germany) and being sort of a cross between Hellhammer and Asgardsrei-era Absurd.

I'd mention Tangorodrim, but I think they suck really hard.

Ride For Revenge is what I'd imagine what would hapopen if some experimental doom band had sex with Von, and the offspring produced collided into Hellhammer. Slow, experimental Black/Death with Tom G. grunts and no electric guitar. However, this band isn't really a straight up clone at all. very different.

Old (of Germany) is a straight up Celtic Frost clone with harsher black metal vox. hell, the first song on their first full length is called Black Jewel throne.

Gravewurm is more straight up Hellhammer worship (maybe a bit on the death metal side, but worship nonetheless). The music can be poorly written at times, but it certainly crushes.

the early (read: good) material of Excruciation is clear and obvious Celtic Frost worship. Not to mention that the band is from Switzerland and formed at about the same time as Celtic Frost...

Baal Sebul is an old Swedish band that (by their own admission) worshiped Hellhammer. Only made one demo that you probably won't be able to find, but do try anyway. The material on that demo is quite good, honestly.

Apocalyptic Slave is a Japanese band that obviously worships Hellhammer.

And that's all folks!

cremator from argentina definitely has a hellhammer vibe with some aura noir

I'm looking for musicians for a Hellhammer/ Discharge worship band called... wait for it... Hellcharge. Yup. Hopefully this will work out, I've got enough material written for a demo but actually need musicians. I can't play drums worth shit.

As I have mentioned every time this gets asked YOU NEED TRENCH HELL. They sound like Venom and Hellhammer.
http://www.youtube.com/watch?v=uWnB1YUlgwI

copy/paste from Black Metal healp-thread

Crude Society System from Sweden:
=> released a 7" in 1985 which sounds like Satanic Rites demo with Apocalyptic Raids riffs

Salute from UK:
=>their debut has an awesome Hellhammer sound, while the 2nd CD is more Frost oriented

Delirium - Zzouuuhh LP/CD
=> Dutch Hellhammer lovers. Incredible dark and atmospheric. Must have !!!!!!!

Hellbastard from UK:
=>Lots of Hellhammer and Celtic Frost (Return To The Eve in particular) riffs

Winter - Into Darkness CD
=> good influence from the song Triumph Of Death, minus the shrieks (deep voice instead)

St.Vitus - Die Healing
=> the guitar sound here is pure Hellhammer. Fantastic. It can't get close except for the next band:

Sheer Terror - Just Can't Hate Enough
=> and only this LP/CD from Sheer Terror, no other stuff of them reached the guitar sound and atmopsphere (If you are into Crust/HC then check out this nihilistic commando names Darkside NYC. Same dark lyrical themes like Hellhammer)

Darkthrone - last song on Total Death is pure Hellhammer worship in the middle part

Raptor from the Netherlands is mostly Hellhammer worship, especially the Necromantic Rites demo.

Hellkommander!
http://www.youtube.com/watch?v=Bd-bgn5jrjk

War Hammer is an obvious clone with a much larger discography.

Also Dishammer. Dischage + Hellhammer = Dishammer

I'll continue this thread necromancy...
@OP I think that in addition to the bands already named, the following covers also deserve a mention:

Both Death and Massacre covered "Aggressor".
Samael covered "A Third of Storms" in a more black metal style.

Also the demo of Nekromantie from Colombia has a slight Hellhammer touch to it, in addition to their not-so-good cover of "Massacra".

Take a look at a new one man band from Germany, called Quintessenz it is pure Hellhammer worship and I think it's very close to the stuff you're searching for. Also check Dungeönhammer from France. All hail the old school black metal madness!