Here are some websites that I have found to be useful for undergraduate music students, but most would also be useful for less-experienced musicians.
A site with dictation with the Philharmonia Orchestra http://www.philharmonia.co.uk/thesoundexchange/make_music/resources/
A site with free copyright-free music http://imslp.org/wiki/Main_Page
A good site with theory and aural information and exercises http://www.teoria.com/
A good site for music theory http://www.dolmetsch.com/theoryintro.htm
A site which compares various aural packages http://www.trainear.com/Ear_Training_Software_Comparison_07_2008.php
An excellent orchestration site from Andrew Hugill http://www.mti.dmu.ac.uk/~ahugill/manual/intro.html
Prof. Phil Tagg's excellent website which covers writing about music and popular music http://www.tagg.org/texts.html
Here is the journal Music Theory Online http://www.mtosmt.org/index.php
Connexions educational website with many usefulmusic modules http://cnx.org/content/search?target=&words=Music&allterms=weakAND&search=Search
Here is a site with Culverhouse Classical Music Collection which you may need to access through your institution. http://www.filmandsound.ac.uk/
Brian Eno's Oblique Strategies for composers http://music.hyperreal.org/artists/brian_eno/oblique/oblique.html
Here is an excellent site for composers with much discussion http://www.compositiontoday.com/
Here are two useful sites on Music Therapy that were sent to me:
Some links to You Tube which have music and the score:
Here is Reich's Clapping Music with a graphic representation http://www.youtube.com/watch?v=eu-tRXgOrdg&feature=relmfu
Ligeti's Piano Etude Cordes a Vide http://www.youtube.com/watch?v=ft_G8bKieNc&feature=related
Ligeti's tape piece Artikulation with graphic representation http://www.youtube.com/watch?v=71hNl_skTZQ
Ligeti's Music Ricercata for piano no. 2 http://www.youtube.com/watch?v=DKCsnCIA7yU
Ligeti's Requiem with the footage from Kubrick's film 2001 a Space Odyssey http://www.youtube.com/watch?v=GPKg2c_bRCs