Ubisoft has released a free version of Assassin's Creed Pirates but you can only play it using Internet Explorer. Don't use IE? You can watch the devs discussing the game - and see some game play in the video at the foot of this story.
According to the blurb, "Internet Explorer and Ubisoft launch the first ever Assassin's Creed title that comes to the modern web with HTML5, 3D WebGL graphics, and JavaScript... New gameplay based on the popular app Assassin's Creed Pirates (ACP) game allows you to step into the shoes of Alonzo Batilla, a young captain, racing his ship through the Caribbean seas, evading mines and other hurdles, amidst searching for treasure. This is also the first web game developed using the Babylon.JS open source framework."