SharePoint Server 2016 Beta 2 (2nd Preview)
Download SharePoint Server 2016 Beta 2 here: https://www.microsoft.com/en-us/download/details.aspx?id=49961
Product key for SharePoint Server 2016 1st preview: NQGJR-63HC8-XCRQH-MYVCH-3J3QR
This gives you a 180 day trial.
Review SharePoint Server 2013 (also applies to SharePoint 2016) system requirements
SharePoint Server 2016 Beta 1 (1st Preview)
Download the SharePoint Server 2016 Preview here: http://www.microsoft.com/en-us/download/details.aspx?id=48712
Not that you would want to spend time on an older preview but its there for reference purposes.
Product key for SharePoint Server 2016 1st preview: NQTMW-K63MQ-39G6H-B2CH9-FRDWJ
This gives you a 180 day trial.