In reply to idiotproof (Buxton MC):
Bach's Well-Tempered Clavier, 1722 - first music to use the perfected (by Bach) diatonic scale, still in use by all western music today, including pop music. Can be said to be the start of modern music.
Beethoven's Eroica Symphony, 1803, is often seen as the first piece of music to start the move from the classical to the romantic traditions. Beethoven's impact on the whole of modern music is equal to or greater than Bach's.
Ragtime and Blues, late 1890s, in America.
Stravinsky, Rite of Spring, 1913, ushered in a new kind of distinctive C20th sound, with its very strong percussion, irregular rhythms, bitonality, discordancy etc. Had a huge influence on earlier (and later) film music.
Jazz, early 1920s, Louis Armstrong etc
Rock and Roll 1950s, well-documented. Several key players.
(I'd like to say that the use of pre-composed pieces by Penderecki in The Exorcist and The Shining had an enormous influence on horror film music scores).