Wikipedia has a bunch of open-source classical music in the ogg format. The quality of the performances has a wide variance. Some of it is pretty good. It almost always needs a bit more reverb to smooth things out. Or maybe that's just me.
The Fulda symphony orchestra has some CD's online here.
Musopen seems to be a central warehouse of open source music. The trick is that typically the music itself has to be in the public domain and the recording has to be "copyleft" or "open source" of some type.
I actually have a little bit of music which could become open source. I should release it that way.
I should at least contribute some of the sound effects I've recorded to freesound.org.
Jamendo has some open source music. As far as I can tell most of it wants you to distribute derivative works with the same license. I presume that these tracks by Piotr Pawlowski are "sample orchestrated" rather than recorded with a real orchestra, but they're rather good.
I'm using some Dvorak to "compose"* some music for Mac Rogers' Universal Robots.
I can't wait to get my violin bow so I can
*I know, I know, lots of people compose with loops of classical music. If I really end up doing something complex I may call myself the "composer". If it's just loops of bits of music then I think we're really only "music editing" here.