Based on an idea I had about 7 months ago, I mentioned it to stampy recently, and he was able to sit down and turn it into a reality! All credit for the implementation and creation of this goes to him! :)
All you need to do is enter the name of each stream you want to watch in the bar at the top, and you can add as many as you want to the page. Whether you want to just watch two, or go fullscreen and perhaps view 3 or even four at once! Even the size of the video and chat box are independently customizeable so you can have each as small or large as you like to fit your own screen resolution and needs. It's even useful if you want to just watch a single stream with a larger video and chat size than livestream offers by default. You can also remove a stream that may have ended by clicking the x at the top right of the embed of it, without having to repopulate the whole page just to get rid of it.
So check it out, I'm sure a lot of people will find it useful! I know I do! XD
(Currently it only supports Livestream streams. Unsure if ustream support will ever be added to it, and justin.tv is pretty unlikely.)