Here on the wiki, we have a YouTube extension installed. You may take any video from YouTube and put it on the wiki to play. However, there are two rules:
YOUTUBE VIDEOS SHOULD ONLY BE EMBEDDED IN YOUR USER PAGE, NOT ON ANY ARTICLE, TEMPLATE, OR ANY OTHER MAINSPACE. Certain YouTube videos can be used as reference links in the reference section of articles, but do not embed the video in the article itself. Additionally, you can only embed up to SIX videos on your user page: any more than this is excessive and may start causing problems for other users' computers.
To include videos, you must take the ID of the video; which can be found in the URL after the v= text, and put it in between <youtube> tags. So for this E3 2007 demo of Super Mario Galaxy, the video ID is G2Rpurw8WG0. Here's what the embed looks like, with the code used:
The width and height attributes can be defined to specify custom dimensions. For example:
<youtube width="300" height="200">qA0vLrwuQHE</youtube>