Other
The Official Leonard Bernstein Site
As the Official site, there are recordings, lectures, letters, photos, publications, scripts, etc. that are unavailable elsewhere. Links to the Library of Congress' Leonard Bernstein Collection, another excellent source.
Classical Net
Classical Net: Aaron Copland, 1900 1990
A good biography, with a good list of works, make this worthwhile, with links to related composers and at least one work, "Appalachian Spring."