P.D.Q. Bach: Sonata for Viola Four Hands and Harpsichord, S. 440
Bax: Sonata in G major; Fantasy Sonata (w/ harp)
Bloch: Suite for Viola and Piano (1919)
Bowen: Sonata in c minor; Sonata in F major
Brahms: Op. 120 #1 in f minor; #2 in Eb major
Clarke: Sonata for Viola and Piano (1919)
Glinka: Sonata for Viola and Piano
Hindemith: Sonatas Op. 11 #4; Op. 25 #4; (1939)
Joachim: Hebrew Melodies, Op. 9
Milhaud: "Quatre Visages"; Sonata #1; Sonata #2
Rochberg: Sonata (1979)
Schubert: "Arpeggione" Sonata
Schumann: Märchenbilder, Op. 113
Shostakovitch: Sonata Op. 147

These are just some of the many viola sonatas - some very popular (Brahms, Clarke, Hindemith, Schubert), others more unusual (Bowen, Milhaud, Rochberg), and at least one that's just plain weird (P.D.Q. Bach - of course). Check out the M 226 section of a music library to find even more.