Not free, but good.  It lets you make loops as long as you want to match the length of your video/slideshow. You can't tell where they patch the loops together.
 http://www.melodyloops.com/music/
I used it for one of my videos http://www.youtube.com/watch?v=YcpTAkU9TaM&list=UU46TdpdufiswReVG1ecOXxQ&index=2&feature=plcp