The Youtube playlist randomizer mentioned before acquired a new feature. It can mix and play videos from several playlists instead of only one.
You can use it to listen your classical and pop music playlist mixed together if you want, but it is much more useful to overcome the built in Youtube playlist limit.
A Youtube playlist can only have a maximum of 200 videos in it. If you have more than 200 favorite videos then you can put them into separate playlists and using this feature you can play those lists like a single, long playlist, so you can effectively have a playlist of infinite length. In theory, at least :).