Beautiful Classical Smaller Pieces, duo,trio,etc

The Julian (Bream) and John (Williams) guitar duo albums are great (Assad Bros. duo also excellent).
Violin/Piano: I like the Prokofiev Sonata #2 (originally flute) and the Saint-Saens Sonata #1.
String Quartets: Debussy or Ravel (Guarneri has a fairly recent high-quality LP of the Ravel w/two movements per side).
String Quintet: Schubert Quintet in C--give me heaven before I die

Excellent chamber music performances in superb sound - some specific recommendations among many more:

Yarlung 95990-787V - Petteri Iivonen, violin, and Kevin Fitz-Gerald, piano: Music of Ysaye, Debussy, Bach and Lefkowitz - Available from: http://www.yarlungrecords.com/180.html#

Yarlung 09261-819V - Ciaramella: Dances On Movable Ground - Renaissance & Baroque dance tunes - Available from: http://www.yarlungrecords.com/180.html#

Philips 835393/4 AY - Schubert Piano Trios, Beaux Arts Trio

Philips 4769648 - Rossini Sonate a Quattro: Nos. 1-6, Accardo, S.Gazeau, A.Meunier, F.Petracchi

Hyperion A66171/A66172/A66173 - The Chamber Music of Malcolm Arnold, Volumes 1, 2 and 3 - Nash Ensemble
.

Quatuor pour la fin du temps ("Quartet for the end of time"), violin, cello, clarinet, piano
(1940–41) Olivier Messiaen

As Tostadosunidos said above, the John Williams, Julian Bream double album is great and one of my favorites in my entire collection. Beautiful guitar work by two masters of the instrument. Its a live recording on RCA and it puts you right in the recording venue. As your system improves, so does this wondrous recording. Here's what you'd be looking for:

http://www.ebay.com/itm/Julian-Bream-John-Williams-Live-DOUBLE-LP-1979-EX-/111690932246?pt=LH_DefaultDomain_3&hash=item1a014c6016

Happy listening ...

Lots of good suggestions above. I am in agreement that the Schubert D. 956 Quintet is one of the truly great works in the chamber genre (the second movement is devastatingly sad and beautiful).

All of the Beethoven quartets, particularly the "late" quartets are also in the "must have" group. All of his other chamber works are terrific. I particularly like his Violin and Piano sonatas (e.g., "Spring" and "Kreutzer").

All of the Haydn quartets are essential too (check out "Hunt" as an example). Ditto for Mozart (check out "Dissonant" for something somewhat strange for this composer).

For something unabashedly romantic, Rachmaninov's Sonata for Cello and Piano is a nice treat.

Now on to more "modern" stuff that is still quite approachable.

One of my all-time favorite pieces is Shostakovich's Piano Trio no. 2. The Beaux Art Trio version is particularly nice, but hard to find except in box collections.

I am also a big fan of the chamber works of mid twentieth century British composers: check out the quartets of Britten, Vaughn Williams, Alwyn and Tippett.

For American works, check out Amy Beach's chamber works.

I like a compilation CD called "Music is Confession" which features the works of Schulhoff, Ullmann, Busch and Kaminski (composers silenced by the Holocaust).

Some of the more modern sounding composers that I like might be a bit less approachable, but, I bet most listeners could appreciate the chamber works of composers like Schnittke and Berg.