Most different band you've heard .

Good point, Windom Pearl.

One very different band would be The Tea Party, prior to the slight change towards mainstream (they still have the lovely "Black Sabbath meets middle eastern folk musicians" sound, though). Nick Cave & The Bad Seeds are different from most I've heard.
When it comes to Metal, though... Andromeda (local band) has a slightly different sound. Their solos are very Symphony X, but they are strangely down-played during vocals... and the vocals are peculiar, but in a good way. Very progressive.

Aside from that... I heard very different bands at a party I attended... it was all synth though, hence very different. :p
You should hear GODKILLER, the progression from each album is pretty amazing. Also ICE AGES, a kind of darkwave/electro band which i like, and BEZERKER who merge techno and metal.
That would be:

Deinonychus - "Deinonychus"

Bethlehem - "S.U.I.Z.I.D." never heard such great riffs and vocals before.

Finntroll - "Jaktens Tid" great band they play a mix of folk,black metal with some polka dance melodies. Yep, original band

Samael "Ceremony of Opposites"

Tiamat "Wildhoney"

Dolorian "Dolorian"

Oxiplegatz "Sidereal Journey"
Most different?

I've never heard anything like the songs on the Cardiacs compilation CD that Lee_B sent around recently, and in a similar sorta way, Devin Townsend's "Infinity" is, with the exception of curious Cardiacs similarities, highly unique in all ways imaginable.

Ulver is truly a band like no other. Do you know anything remotely like "Themes from William Blake's..."? I know I don't.

Finally, Frank Zappa rules the notion of "different" from the perspective that not only do several of his albums sounds like nothing else ever made ("We're Only in it for the Money" comes to mind...) but his albums also sound nothing like each other! Freak Out, WOiiftM, Hot Rats, Burnt Weenie Sandwich, The Grand Wazoo, 200 Motels, Overnite Sensation, Roxy and Elsewhere, Joe's Garage, and The Yellow Shark (some random examples) all sound completely unique from each other from style to composition to production.

(and, he even wins out for the best song titles, beating even Anal Cunt, who made it a major part of their schtick. "Prelude to the Afternoon of a Sexually Aroused Gas Mask"? Beat that...)
I'm going to have to agree about the Cardiacs on this one. Also with Nick Cave.

And after seeing Dancer in the Dark, I've gained a lot of respect for Björk, and her bizarre vocal style.
The Tea Party - different because of the varying styles (such as Middle Eastern) inflected into their music. Also, by far, my favorite non-metal band of all time.

When Led Zeppelin first came out, they were by far the most different band I heard at that time - looking back, I still feel they were 10-20n years ahead of their time.
and PORE (find their demo dammit! Its hard to find though, came out in 99...french band) Very wierd mix of Black metal, Techno, Dance, dark ambient, neo-classical shred, death metal and grind....