This was released back in March by a Source Filmmaker group known as Deviant Pictures Films. Aside from the odd decision to make Alyx look completely
different, this is actually a really well made trailer. Yeah, some of the assets lack polish and don't mix well with others thematically, but the animations are superb and it's pretty intense. If a small group of fans can make a trailer this good, then I'm curious to see what Valve will come up with.
Can't get the Youtube tags to work properly. Dunno why.