Beautiful Classical Smaller Pieces, duo,trio,etc

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.

YES, the Villa-Lobos quartets are beautiful and there are a lot of them. Brilliant had a really nice and cheap box set for the complete quartets.

Schubert,

Janacek #2 IS a terrific modern piece, and # 1 (Kreutzer) is worth mentioning too.

For modern American I would add Carter's Quartet #3 and Ives #2.

For another "modern" composer, but, composing more in a late romantic style, Schoenberg's "Verklarte Nacht" is a basic item.

I know this is intended to be a basic list, but, can we add mention of obscure or odd-ball items that are worth tracking down? In that context I would throw in Somei Sato's "Birds in Warped Time II."

I like the three string quartets by the Spanish composer Arriaga who did not make it out of his teens. I believe he was 16 years old when he wrote them.