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, although further information is available by following the links to the left. In order to listen to, view and print scores using Sibelius Music, you will need to download and install the free Scorch plug-in. This bit of software also allows you to transpose some of the pieces for different solo instruments and also comes in handy for other music websites.