“Art of the States” is an interesting website, showcasing American composers and performers, and providing streaming audio (in RealPlayer format) of many works. You can search by composer, performer, instrumentation, time period, or genre. There is also an extensive list of links related to composers, performers, labels, and publishers (it is somewhat buried, under “About Art of the States>>Resources”).
The site also offers some interesting feature presentations, which cover some aspect of music in the collection. For example, the most recent feature offers composer John Harbinson providing an overview of his String Quartet No.3. This item dates from May 2008, so it appears that updates are infrequent. RSS feeds are offered on the website, allowing users to subscribe and be notified with updates when new content is added to the site (for a good intro to using RSS feeds, see this link).
One feature of interest to composers and performers is that the site accepts submissions, for inclusion on the website, and for distribution via broadcast radio networks. Note that Music Dept. faculty Fred Lerdahl and Arthur Kampela have works included on the site. Take a listen!