Tableau 6.0 is now available to everyone with current maintenance. You can download it from the Customer Account Center. This is a major release with a large number of new features in both Tableau Desktop and Tableau Server. Learn more about What's New in Tableau 6.0.
Also available is Tableau 5.1.8 and 5.2.5. You can download these versions from the Alternate Download Site. A list of changes in these versions can also be found on the Release Notes page.