A new Doctor Who mini-episode has been released in the U.S. "The Battle of Demon's Run: Two Days Later" tells the story of how the Sontaran, Strax was resurrected after the events of "A Good Man Goes to War". In that story, Strax apparently died at the end, but he was alive again and working with Madame Vastra and Jenny in "The Snowmen". That story contained only a brief mention by the Doctor that a friend had brought him back. "Two Days Later" fills in the missing gap between the two stories.
Unfortunately, the three-minute episode is not currently available outside the United States, where it can be downloaded from Amazon for free. It was briefly up on Blogtor Who, but has since been taken down. However, according to Blogtor Who, it will be released in the UK shortly. I have no idea if this includes other areas of the world, including Canada, as well (despite mentions on the Doctor Who News Page that it's available in "North America", it is definitely blocked from download in Canada).
It's an odd decision to release this at this time, out of order and just before a new episode not involving Strax is about to air. It would have made more sense to have released it closer to the broadcast of "The Snowmen", perhaps as one of the many prequels that story had. The episode itself doesn't really reveal very much, just that Vastra and Jenny healed his wounds, but not how. It is implied that Strax didn't actually die, just came very close, which makes more sense than a resurrection, but does contradict statements in "The Snowmen". Most of the episode is played for laughs, which seems to be Strax's lot now as a comic-relief character. I still find that a bit of a shame as he was a much more complex character in "A Good Man Goes to War". Oh well.