Robert, you are probably aware that "little known" will eventually (as in right now, in this post!) raise the question, "little known to whom?"
So I'm asking, to whom are Sullivan, Scarlatti, Cimarosa, Berwald, Stenhammer, and Tallis "little known"?
Otherwise, I'd suppose that most music of the past hundred years or so is little known to most people who call themselves classical listeners.
If anyone here, for instance, knows the music of any of these very prominent composers, I would be very surprised and vastly pleased:
Ashley
Bruemmer
Childs
Dumizio
eRikm
Ferrari
Gobeil
Huber
Karkowski
Lachenmann
Mumma
Normandeau
Ostertag
Paik
Rowe
Smalley
Tone
Uitti
Yoshihide
Zanesi
Lachenmann and Ostertag are in my collection. And although I had heard of all composers posted so far in this thread by Robert Newman, and many are in my collection, I think that the ones selected still are mostly interesting choices and can help people look beyond the established grandmasters.