We've had a few occasions where multiple threads have been started showing the same video clip. It's a bit of a pain to keep on top of this and therefore I'm starting this thread where you can post the videos. I'll start it off with some of the popular ones from recent times. This thread will also be linked from the FAQ thread.
Before you post something, check here in case it's already been posted. You can also do a search across the whole forum by entering the youtube reference in the search box (this is not foolproof, as some videos are posted a number of times on youtube)
If someone starts a new thread we'll transfer it over here or close the thread if the video has already been posted