I find it incredibly annoying when an embedded YouTube video can’t be made fullscreen, and I have to switch to YouTube.com just to watch it.
So, I wrote this simple little bookmarklet which modifies the embed code for any YouTube videos to allow fullscreen. It could also easily be made into a user script for GreaseMonkey, etc, to perform the modifications automatically.
Try it out on this page (I’m too lazy to actually make it fullscreen).
Note that currently it doesn’t actually check to make sure it’s a YouTube video that it’s modifying, so it might stomp all over other types of embeds.