SharePoint 2016 RTM and keys are available for downloading in MSDN!
SharePoint 2016 in MSDN
It is not officially anounced yet. I think the official anouncement of the general availability will be in a few hours on The Future of SharePoint event. If you haven’t registered for it yet, I would recommend to do it. Jeff Teper, the father of SharePoint, will unveil Microsoft vision and roadmap for SharePoint and OneDrive for Business on this event.
SharePoint team has just anounced SharePoint Server 2016 Beta 2 and Project Server 2016 Beta 2!
This is the last public preview of SharePoint Server 2016 before General Availability which is expecting to be in Q1 of 2016. Upgrade from SP Server 2016 IT Preview is not supported and you need to install beta 2 and migrate IT Preview content to the new Beta 2 deployment.
Announcing SharePoint Server 2016 Beta 2 and Project Server 2016 Beta 2
SharePoint Server 2016 Beta 2 (download link)
SharePoint Server 2016 Beta 2 (technet resources)
If you stumbled into some issues with this beta, you can find help on this forum: SharePoint Server 2016 Preview (Technet forums)