This extension allows you to add youtube and googlevideo to Expression Engine forum posts. To use it in the forum a user just needs to add:
in their post. There are two functions included. The first one is to get the code translated into the correct html object code and the second one adds some user friendly buttons to the forum post interface. Here you can see it working.
Expression Engine Forum Module 1.3.2 is required in order to run that extension.