I’m thinking personally of when Thurston from Sonic Youth did some songs with the “supergroup” Twilight. What are some examples of artists outside of the metal genre that come in and do a guest spot on an album. Are they a fan of the band in question or do they have some cultural commonality like Diamanda Galas when she collaborated with Rotting Christ?

Sunn O))) has a few. They have collaborated with Nurse With Wound, Ulver (they used to be metal, of course, but not when this collaboration was made), and Scott Walker.

On a more infamous note, obligatory mentions of Ice-T and Black Sabbath on a song on "Forbidden" and Lou Reed and Metallica on "Lulu".

Jarboe of Swans fame collaborated with Neurosis, also there are Julie Christmas album with Cult of Luna and Emma Ruth Rundle with Thou... Post metal is an hotspot for those!

My favorite is probably the collaboration the Wardruna guy with Ivar Bjornson of Enslaved, both albums are really interesting and well done

One of the interesting ones I can think of now is the collaboration between Yukihiro Isso a Japanese flutist with a black metal band called Magane.

Their collaboration resulted in a bonkers album called Quest for blood.

https://www.youtube.com/watch?v=vmrin4qOOFs

In general, Isso's music is great, check him out.

I'm also rather fond of the infamous Lulu collaboration, but from the standpoint of someone who doesn't care the slightest about metallica, but likes Lou Reed's output.

I wouldn’t necessarily say Julie Christmas is a non-metal artist, given that before she became a solo artist she was in Battle Of Mice. Still a killer collab though, of course!

Noise-artist Merzbow has had a fair amount of collaborations within the metal community, most notably his frequent collaborations with Boris and the couple of projects with Full Of Hell. Speaking of Full Of Hell, they also have that recent collaboration with electronic/industrial/whatever group Health. I wasn’t too big on the song when it was released as a single, but I actually really enjoyed Health’s album for the most part and liked the song when hearing it as part of the overall project.

EDIT: Also somewhat of a less apparent example is David Tibet from weirdo neo-folk group Current 93, who has done spoken-word passages with certain black metal bands like Sigh and Skitliv. For an artist that has a lot of Christian themes in their music, albeit rather esoteric and gnostic in nature, there is a certain unsettling and disturbing darkness in Current 93’s lyrical themes that fits like a glove with black metal. There is a surprising amount of Gnostic themes I’ve seen popping up in metal as of late in general, but that’s a different conversation.

There was Sivert Høyem on Satyricon's S/T.

Didn't Mizmor recently do a collaborative EP with Scott Walker too? Haven't heard it but I imagine it's good.

My bad! It was a collaboration between Mizmor and Andrew Black. The EP is post-rock/ambient according to RYM, sadly not metal.

Julie Christmas is a metal artist though, she was in Battle of Mice, etc.

Steven Wilson is a prog / rock guy but he's had an incredibly close partnership with Mikael and Opeth for years. Their team-ups in the early 2000s coincided with both of their bands reaching newfound success in their genres.

I really liked Ihashn's collaboration with a fiddle player called Knut Buen in a project called Hardingrock. Every once in a while I'll remember it and go back to it.

body count isn't on the archives (did they use to be?), but i'm not sure ice-t really counts by the time forbidden came out

Anthrax had a few of these. They collaborated with and toured with Public Enemy, and also worked with Beastie Boys in some capacity. At the time, both of these bands were huge household names.

James Hetfield has worked with Primus and Heart, and I am sure a few others that I have since forgotten about.

I second this one too. Quite fun...

Keep of Kalessin did some performances with Alexader Rybak, a Norwegian pop musician who won the Eurovision Song Contest.

Kari Rueslåtten (from the Fenriz/Satyr project Storm and The Third and the Mortal) could maybe count here, as she has mainly done non-metal work.

The Judgement Night soundtrack is full of non-metal artist collaborating with metal / rock artist.

Click at own risk.

https://youtu.be/Nryo91pHK8s

Sakari Kukko is a Finnish multiinstrumentalist known for his jazz/folk/hard-to-describe band Piirpauke, and he's done a lot of work with Amorphis - though when the collaboration began, Amorphis weren't playing that much metal either.

According to the band's authorised biography, Kukko had some weed to sell to the band's then-singer/lyricist/killjoy Pasi Koskinen, and as a result they hit it off well enough for Koskinen to ask Kukko to guest on Tuonela. This probably counts as cultural commonality

The Scott Walker collab is so fucking weird. Normally Sunn O))) seem to take the lead on their collaborative albums and the end result is pretty clear-cut drone or dark ambient records, but Soused just sounds like a Scott Walker album with weird-sounding guitars.

Sadly Phoenix is the only good track on that whole album. Would love to hear a bigger collaboration between them in the future because I really do love that song.

So no one's mentioned this collaboration yet so I'm throwing this into the conversation. I'm honestly not surprised because neither of the artists in question are particularly well known (as far as I know), but a Raw Black Metal/Noise project called Sutekh Hexen released an EP called "Breed in Me the Darkness: the Second Coming Mixes by Andrew Liles". Obviously, even though this isn't a full-on collaboration I'd still say a remix album is close enough. The remixed tracks are among the weirdest metal tracks I've ever heard, you need to check this out- along with the label that released this, Aurora Borealis. Andrew Liles adds so much to these tracks, I've yet to hear anything like them.

Just remembered a funny one: Xzibit with Within Temptation!