Alright, this is really easy.
Most all forums use what is called "BBCODE" to make a lot of things work, such as bold and italics, inserting links, etc. I have added some youtube BBCODE to the forum.
For example, here is a link to a Youtube video:
http://www.youtube.com/watch?v=3XsI2iGDjVg or
http://www.youtube.com/watch?v=3XsI2iGDjVg&feature=related
All you need is the code that comes after
v= but before the
&crap in the URL.
This is not needed:
http://www.youtube.com/watch?v=
This is what you want, the video ID:
3XsI2iGDjVg
This is not needed:
[noparse]&feature=related[/noparse]
Now then, when you have your code, simply type this into the editor window:
[youtube]3XsI2iGDjVg[/youtube]
... and you'll get this!
Yeah, I know, there isn't some easy button to click 2-3 times and paste the URL, but it's still more or less foolproof.
Embed away!
NOTE:
Some Youtube videos start with - or _ and are not alphanumeric video IDs.
For example, -as7eth6w5 or _ser_9j5we could be an ID, so watch for those header characters.