I.e. Not an electric guitar + bass + drums (the typical metal assortment imo).

I was thinking along the lines of black metal which seems to be one of the most malleable metal genres. Gris uses some string instruments and piano such as the last track on the album Il Était une Forêt... Also, Ulver's Kveldssanger.Both beautiful albums. More so, Ulver.

I'm curious to know if their's any more albums that incorporate more the way Ulver did where the entire album was mellow with a cello and an acoustic guitar (suggestions don't have to be exclusive to those instruments). Or anything you see interesting. Doesn't have to fit what I recommended to a key. Just as long as it has a-typical instruments.

Well, there's quite a bit of that in all extreme metal genres. Piano, choir, and strings are fairly common (off the top of my head, Swallow the Sun, My Dying Bride, Agalloch, Dimmu Borgir). Then there's the entirety of the Folk metal genre (Eluveitie and Finsterforst are my two favorites), they use everything from accordions to hurdy-gurdies. But for some extremely interesting use of atypical instruments, try Unexpect: http://www.youtube.com/watch?v=5XlBGMAsT3o

EDIT: I can't believe I forgot this, but Pantera made a song with a what I'm pretty sure is a cowbell. Yeah. http://www.youtube.com/watch?v=2x_PDfkQUfw

Chthonic plays symphonic black metal with Asian instruments:
http://www.youtube.com/watch?v=e_UkowmQs30

There's also another band (can't recall their name) which uses it even more and is mainly instrumental.

Botanist's debut album has no guitars, but a hammered dulcimer instead. Don't know if that'll interest you..

Whoa, Botanist it trippy...and by the way, if anyone happens to know of a metal band with a didgeridoo, I will buy you an internet beer.

A thread like this crops up fairly regularly, maybe we could do with an "unusual instruments" thread?

Anyway, my usual rec for this topic is Noxagt; Norwegian instrumental sludge/noise-rock with distorted viola instead of guitar on their 1st two albums:
http://www.youtube.com/watch?v=rqcJTKm6upY


As for didgeridoo, not quite metal, but fairly close, a band I recommend at every opportunity: Crow (UK):

http://www.last.fm/music/crow+(kent)/_/Thunderbird
http://www.youtube.com/watch?v=3lg40lwzOiA

I also remember Hate Eternal having a didge in one track off "I, Monarch", but I can't remember which.

I think Fejd truely managed to be a acoustic metal band. The drums help a lot for this too.

This Sear Bliss song is one of the best examples that immediately comes to mind, particularly because of the muted trumpet part http://www.youtube.com/watch?v=CsCIP-jw1Oo
Most examples of horns in metal are synthesized and have no real character. This one on the other hand, though short, does a lot in terms of atmosphere.

Oh that's the band where they literally play in the woods. More black metal bands should do that just as a general rule of thumb

Are you thinking of Quest for Blood?
http://www.youtube.com/watch?v=HPUPAkP12QM

Rotting Christ use a gaida on the track Nemecic, and Sigh use a bunch of weird instruments and effects too.

Thank you, good sir

Blood Ceremony has a flute.
http://www.youtube.com/watch?v=am2fcSNTWOs

There are some unusual instruments on some Taake songs.

Banjo solo!
http://www.youtube.com/watch?v=tj_CwKxZMno

Alchemist.

http://www.youtube.com/watch?v=Q9YcPuCv0aY

Wow. I am currently nursing a small erection. I need this album.

Try and listen to Pan.Thy.Monium http://www.metal-archives.com/bands/Pan.Thy.Monium/338