Schubert's Cello Quintet of course.
Of the top of my head;
Brahms Sextet No. 2 in G Major, Op. 36 (Sextet for Strings)
Beethoven's String Trios
Of the top of my head;
Brahms Sextet No. 2 in G Major, Op. 36 (Sextet for Strings)
Beethoven's String Trios