In the course of a lively critics' conversation at ArtsJournal, Terry Teachout recommended something called Pandora, which is supposed to "help me discover more music that I'll like." You enter the name of an artist or song, and it suggests more music in the same vein. I'm afraid I had bad luck with the device on my first few tries. I typed in "Mahler." It said:
We found several. Which 'Mahler' did you mean?
— The Mausker, by Deerhoof
— Masher, by the Mountain Goats
— Madner, by Underworld
— Madder, by Groove Armada
I then typed in "Debussy." It said: "Do you want 'Debussie' by Daphne Loves Derby?" Not right now.
Update: Maryann Devine has queried Pandora on this issue. She was told that "classical music is so complex that they haven't been able to incorporate it into their algorithm." Mm-hm. To play fair, I entered "Simple Twist of Fate," my favorite Dylan song. I got Neil Young's "Thrasher," another all-time favorite, together with other songs from Blood on the Tracks and a bunch of strummy stuff I didn't immediately care about.
Update 2: Ace young composer Timothy Andres points out that the similarly constituted Last FM site has assimilated notational music, up to a point. Type in "Mahler," and you receive recommendations ranging from the obvious Bruckner to the less obvious Leadbelly to the one and only Дмитрий Шостакович.