Microsoft SQL Server has been in developer edition as far back as 2005 (if my memory serves me); however each license was $50.00. Now with SQL Server 2014 & 2016, Microsoft has made developer edition free.
There is no better time to get on board with SQL Server 2016, there are lots of new functionality and features available. That can help your organization reach new levels of performance, scalability, and data insight.
Download and start playing with SQL 2014 here.