Just remember that royalty-free doesn't necessarily mean FREE. There are many free tracks on these sites, but many also require a fee. But it's a one-time fee. That's why they're royalty-free.
You'll also find a very nice library of royalty-free FREE music at Premiere Elements support site Muvipix.com. Their royalty-free music is also free -- but much of their library is only available to subscribers to the site.