The second revision of Star Fleet: A Call to Arms, the classic game of space combat set in the Star Trek universe, is available PWYW via DriveThruRPG [AL] and RPGNow [AL] for a limited time. As explained on the product page, this has been done to allow those who have purchased the original edition of the game to gain access to the newer updated version of the game. It also allows those who have been interested in this game to view it and decide if it’s something that they want to invest time and money in. While this book is focused on the rules of playing the game, it’s also a great resource for anyone playing a Star Trek-based game, as it provides details on many ships that can help to build your own in your system of choice.
Updated November 18,2014: Article title corrected to mention Star Fleet.