Musopen has a ton of classical music. Many of their recordings are public domain (CC0 if memory serves), others are under various CC licenses. All the underlying compositions are public domain.

ccMixter is another great resource.

If metal's your thing, check out Open Metalcast. I particularly like their instrumental episodes, called instrumetalcasts.

There's a good amount of CC-licensed music on Bandcamp, but it can be hard to find - there's no way (AFAICT) to filter out non-CC music.