A couple other people already linked it, but the Bad Lip Synching video for Game of Thrones is pretty cool. Or at least parts of it are. It takes the form of a trailer for an imaginary movie about a bunch of people trying to set up a Renaisance Faire. A lot of the bits don't really fit in that well with the theme though.
There's also a new CGP Grey video out, about how many countries there are in the world. If you haven't watched any CPG Grey videos before you should watch this one, and then go watch some more.
A couple weeks ago i linked to the Honest Trailers/How It Should Have Ended mashup for Star Trek Into Darkness, but now How It Should Have Ended has their own video for it. I like it of course because it covers the big things in those movies that i like to complain about :)
Meanwhile Honest Trailers has done a new video for the Matrix, and those other two movies. I don't have the hate-on for the Matrix sequels that some people do, but i still find such jokes amusing.