This post is an ongoing collection of YouTube-based tools for making split-screen media. These web-based tools allow you to select the YouTube clips that you want to use as part of your own multi-channel mashup. I'll update this list as I find more; newer additions are nearer the top of the post.
YooouuuTuuube lays out the frames of a YouTube clip within a grid. The frames can be scrolling by as the clip plays, or be shuffled around the screen. Examples: Alice and Auto-Tune the News #2. See the related MetaFilter thread for more samples.
YouCube creates an interactive 3D cube whose faces are composed of YouTube clips. You can rotate the cube to control the visible faces and the audio mix. See the demo video for YouCube.