The music of every composer that died before 1942 is royalty-free and in the public domain. (70 year rule). The same doesn't go for actual recordings, since the artists making the recording usually demand royalties, except of they have expressly reneged on them. I found that a lot of royalty-free music can be found via wikipedia and the composer pages there.
Also the site
www.musopen.org has lots of music. Among other from public orchestras that cannot or will not enforce royalties (US Airforce Band did a couple of Holst pieces!)
I just uploaded a vid to youtube with expressly public-domain music. It was recorded by a pianist who expressly put her music in the public domain, playing a piece by a composer who is royalty-free. IMMEDIATELY the video was hit with automatic copyright claims by EMI etc... I had to enter a dispute and present the evidence. I really find it terrible that these companies try to enforce copyright just because they made a recording of the same piece, but by another artist...