There are many really good high definition sex sites available in true 1080 hd today. The number of porn videos and movies available at each site will vary on how long they been using a hd camera and how often they update their websites.
With all porn sex sites, they all offer free high definition sex scenes that you can watch for free. These sex vids or clips if you want to call them that are viewable through free hosted galleries or at their site themselves to give you a preview of what you can expect to view after you purchase a membership id.
Many of these videos are getting longer and longer and really give you everything you need to see in the sex scene. For a lot of people that is enough. For others, it gives them the incentive to buy an id and get full access to the entire scene.