It feels like a bit of an insult to relegate some of these to background music, but to pick the most obvious composers:
- Chopin: Nocturnes & Mazurkas
- Mendelssohn: Songs Without Words
- Schubert: Impromptus (especially the Gb major)
- Slow movements from sonatas by Haydn, Mozart, Beethoven, etc
- Bach: Inventions & Sinfonias; some of the WTC (esp. the Book 1 C major prelude); Air on the G-string
edit: I forgot the French contingent:
- Satie: Gymnopedies, Gnossiennes
- Debussy: Clair de Lune, Arabesques, Réverie, ....
- Ravel: Pavane Pour Une Infante Défunte
- Fauré: Pavane, Sicilienne, ...