Sketches and manuscripts.

Playing from printed music can sometimes be an anonymous experience. Black and white staves covered with uniformly round circles, neatly arched slurs and pin-point-perfect dots. Enough to make anyone forget that a human being once put pen to paper or that written scores form only a small part of the creation of a musical performance. In the written tradition of classical music, scores are invaluable, if far from infalliable musicological sources. Thus, for both performers and scholars, the unveiling of the Julliard School of Music's swish new web-site that makes a vast store of manuscripts, scores and sketches available for free online, is a noteworthy moment.

p.s. take a look here at The Rest is Noise, a blog by the music critic of The New Yorker for more details.