The Nico Nico Douga website has just begun yesterday to stream all the video anime of Black Rock Shooter in order to promote the upcoming OVA. These videos will be streamed until August 31.
Here's the link:
http://ch.nicovideo.jp/channel/ch961
Although you can also get the trailers and previews here:
The soundtrack is one of the most badass things I've ever heard:
Looks friggin' sweet! :-)