The Vienna Octet
"Beethoven: Septet in E-flat op. 20" (Vinyl)

One of the more oddly-shaped ducks in Beethoven's canon, the septet (scored for clarinet, horn, bassoon, violin, viola, cello and double bass) helped a young Beethoven gain popularity.  Its form is unconventional, resembling a serenade rather than a traditional four-movement work and it is a work of substantial length (also uncommon for the medium.)

This composition was re-arranged by Beethoven himself into his opus 38 piano trio and later Toscanini had a chamber orchestra perform with similar instrumentation.

All of this history aside, I've never heard this composition before today.  It's a youthful work from the revolutionary composer and its unusual shape has probably ensured it rarely makes it to the performance market.