If you always see YouTube videos in full screen, this is something that interests you greatly, although it only works for Safari users as it is an extension of it.
What FullTube does is very simple: all YouTube videos are shown in full screen without asking, something that for some can be highly intrusive but that will save others hundreds of clicks in the long run, I'm sure.
You obviously need Safari 5, as it is an extension of it.
Link | MuchUseful