I think the original request was for 'essential' chamberworks of the 20th
century, and I think by now that request has pretty much been fulfilled.
So, I will just add a few non-essential items that I particularly enjoy
and you might want to give them a try sometime.

Gorecki's 1st & 2nd string quartets.  I'm not a major Gorecki fan, but I
love these very sparse and chilling quartets.  The version I have is played
with great technical finesse and a lot of attitude by the Kronos Quartet.

Robert Simpson's Horn Trio and Horn Quartet. I'm a sucker for horns.

Schnittke's Piano Quintet.  Actually, it's a live performance of this that
I remember so well that just left me spellbound as the piano plinked away
into nothingness.

Rochberg's 3 piano trios.  I've only listened to this CD once and I can't
remember why I liked them so well.  I just remember I did.

Bloch's 1st and 2nd piano quintets.  I especially recommend these for those
sensitive souls who get so upset by 20th century music and fear that it
portends the end of civilization.  These are simply beautiful....with a
modern edge.

John Parker
Tucson, Arizona