Alright folks, I typically LOVE when a band releases an experimental album/ep that is not typical of their style.

Some examples are:
Moonspell's Sin/Pecado
My Dying Bride's 34.788%... Complete (some would say Like Gods of the Sun too, which I love as well)
and Finntroll's Visor om slutet EP

I want the good and the bad, since the albums listed don't normally get favorable reviews. Thanks in advance!

Broken Hope - Loathing, for such an unexceptional band Loathing was very unusual and awesome

Great mention! Quality and style-wise, very night and day compared to their preceding albums.

Diabolical Masquerade - "Death's Design".

Morgoth - Feel Sorry for the Fanatic
Paradise Lost - Host
Danzig - Blackdevil
Massacra - Sick
Kreator - Outcast / Endorama

this one's only half an album, but defeated sanity's last album was a sort of a split with themselves, where the first half is their usual bdm, while the latter half was a (very well done) tip-of-hat to the 90's tech death bands, like atheist, cynic and death

Anal Cunt - Picnic Of Love

Some that come to mind:

My Silent Wake - Invitation To Imperfection
Alcest - Shelter
Becoming The Archetype - Celestial Progression
Lengsel - The Kiss, The Hope*
Virgin Black - Requiem - Pianissimo

*They only have two albums... but this one is waaay different than their first

Well theres Cryptopsy's abortion of an album we don't speak of, Dismember has Massive Killing Capacity, their only album which isn't straight up DM but tried doing some of the death n roll like Entombed were doing, Testament had Demonic which was their most DM album.

I'd say it's also more of an old-school sounding stuff then Defeated Sanity's usual style, which is BDM, but more of a technical technical (or at times maybe even progressive) kind.
On that release they moved all the prog to the second part, leaving first one more direct, downtuned and just brutal IMO.

On the subject: if we're talking late 90-th here, the time of experiments with new genres - which is what PL "Host" and MDB "34.788%... Complete" were, then we could probably mention more legendary bands with their trend breaking albums.

Like "Haven" by Dark Tranquillity - slower, way more "groove" influenced and somehow warmer and consistent sounding than their previous swedish melodeath releases, as well as their subsequent modern-ish MDM.

Like "The Great Beast" by Mystic Circle - same pattern as with DT's "Heaven" here - slower, groovier, warmer, consistent.

Like "Diabolus in Musica" by Slayer. Man, this release got a lot of flack - but I love it! It's a more complicated story than previous mentions because there's so much going on on this album, but there definitely is ton groove here, more than on any other Slayer release IMO.

Like "Diatribes" by Napalm Death and "Demonic" by Testament - since we talk groove. Or Morta Skuld "Surface", though this is way less known stuff.

Like "Anguish - I harvest" by Sacred Sin. More on it here or here.

Like "The Haunting" by Sear Bliss.

And probably many more releases like that that just don't come to my mind right away.

But not all was groove in late 90-th.

There was also "Skycontact" by Phlebotomized. No comments

Then there was "Medieval Rites" by french band Agressor - and this one is in a way like "Diabolus in Musica" sans groove though. There's A LOT going on in this release, which steps beyond the usual old-school death/thrash Agressor performed on other releases.

And then there are debuts. Starting from Darkthrone and going on for a ton of bands (including aforementioned Paradise Lost, or not yet mentioned Amorphis etc) that had significantly or even drastically different debuts than the rest of their catalogue. But let's not go there, because this kind alone can fill up this topic to the brim.

And as for Amorphis - I'm surprised nobody brought up their very interesting (though not to closeminded kind of metalheads ) EP "My Kantele".

P.S. Oh, and how could I forget the most unusual "Odin ownes ye all" by Einherjer?

You wanted bad?
Celtic Frost - Cold Lake

If you tolerate non-metal bands, Bad Religion's second album "Into the Unknown" is in progressive rock vein.