Star Trek: Axanar is probably one of the most highly-anticipated, professionally-produced fan films currently in the works. After the release of an official trailer back in June, a short prelude film, Prelude to Axanar, come out earlier this week.
The docu-drama style short introduces us to the world of Axanar, which is set some 20 years prior to TOS during a time of war between the Federation and the Klingon Empire. It's a race for technological and space superiority, with the fate of the Federation itself hanging in the balance.
The film can currently be viewed as an embedded video on the Axanar Kickstarter page. Scroll down to the second video from the top.
UPDATE: the film is also now available in HD on the Axanar YouTube channel.