I've found a couple of "series" on YouTube that I love. One is called Skyrim for Pimps, which a couple of guys just play the game and comment on it. After enoying the hell out of that, I found more of their stuff and really liked it. Their user account is GameSocietyPimps.
Here's my favorite segment in season 1
I'm also a big fan of HISHE (How it should have ended)
and some other Machinima stuff. What about you guys? What have you found worth watching on YouTube?