Media Shuffle is a simple, free application that allows you to play multiple media files from a large collection at random. Just point the app to a root folder of sub-folders containing your media file collections, and it provides you with a simple menu to pick and choose which files you're in the mood for. It will pick from your chosen folders at random (or sequentially, or at random then sequentially, it's up to you) and launch as many files as you request in your chosen media player, to be played back-to-back. If you use the free VLC media player, then you'll have the option to play the files in loop mode.
I like this program, it shuffles all of my .AVI files from my external hard drive but it doesnt select my HD .MKV or .MP4 files. If you added that, this would be exactly what I was looking for.