Alert reader Marc Weidenbaum (web site here) found the PDF files of the Beethoven piano sonatas for me here at the Sheet Music Archive. This excellent little site also contains music by:
Albeniz / Bach / Bach (C.P.E.) / Balakirev / Bartok / Beethoven / Bellini / Bizet / Boccherini / Borodin / Brahms / Chabrier / Chaminade / Chopin / Clementi / Couperin / Czerny / Debussy / Dvorak / Elgar / Faure / Field / Franck / Glazunov / Gottschalk / Granados / Grieg / Griffes / Handel / Hanon / Haydn / Henselt / Joplin / Kalinnikov / Liadov / Liapunov / Liszt / Macdowell / Mendelssohn / Moszkowski / Moussorgsky / Mozart / Paderewski / Paganini / Ponchielli / Prokofiev / Purcell / Rachmaninoff / Raff / Rameau / Ravel / Rossini / Rubinstein / Saint-Saens / Satie / Scarlatti / Scharwenka / Schubert / Schumann / Scriabin / Sinding / Stravinsky / Tschaikovsky / Verdi / von Weber / Wagner / Widor
Enjoy!
How about a Postclassical such site?: imagine someplace where you could download scores by Mikel Rouse, Rhys Chatham, Eve Beglarian, Janice Giteck, Glenn Branca, William Duckworth, Maria de Alvear, Elodie Lauten, Bernadette Speach, Art Jarvinen, Kyle Gann….