Sibelius Music
Sibelius Music is a great source of music for all sorts of combinations of instruments and a site where you as a composer can publish your own pieces or other musical resources. In looking for interesting music you have to be prepared to sort through some less useful things; the site is open to anybody, so anyone can publish anything, with the obvious result that there is a fair amount of rubbish there as well as the good stuff. You can find my pieces and arrangements at my Sibelius Music Page