Stanford subscribes to several score databases containing standard Western music literature in full score, performing editions, and orchestral parts.
In addition, the following source is freely accessible on the web:
IMSLP provides downloadable public domain (i.e, published prior to 1924 in the US) scores. Image quality is uneven, and accompanying information is sometimes incomplete; proceed with caution. IMSLP was recently reviewed in the Journal of the American Musicological Society.